iOS App uses SID instead of LDAP username

I want to migrate from ownCloud to nextcloud. I just installed nextcloud on a new VM.
Excactly as the owncloud Installation the nextcloud should be an Access Gateway for companies internal SMB share on a Windows Server.

System Details:
ubuntu 18.04
nextcloud 18.0.5
Apache2
mysql Ver 14.14 Distrib 5.7.30
nextcloud app: external storage support 1.9.0
nextcloud app: LDAP user and group backend 1.8.0

external storage authentication Parameter: Log-in credentials, save in session

After setting up the VM with nextcloud and configuring the external storage and ldap Integration, I have the following issue:
If I log in with my ldap credentials from a Windows Computer via Webbrowser, external storage works fine and everything is okay.
But if I log in with nextcloud app from my iPhone, I get the message “Error 503 - Storage is temporarily not available”

The app authentication submits the wrong accountname (SID instead of LDAP user name) - see screenshots from eventlog of Windows File Server below.
2020-06-11 13_21_30-Clipboard

Screenshot while opening smb share via Webbrowser (Microsoft Windows) - everything works fine
2020-06-11 13_22_58-Clipboard

Complete message in /var/www/nextcloud/data/nextcloud.log
{“reqId”:“eujl7n8kkTUbFQmIdhdG”,“level”:4,“time”:“2020-06-11T11:01:10+00:00”,“remoteAddr”:“185.22.142.203”,“user”:“55D61FCD-423D-44BB-ACAF-8951EB377A98”,“app”:“webdav”,“method”:“PROPFIND”,“url”:"/remote.php/webdav/INSTALL",“message”:{“Exception”:“Sabre\DAV\Exception\ServiceUnavailable”,“Message”:“Storage is temporarily not available”,“Code”:0,“Trace”:[{“file”:"/var/www/nextcloud/3rdparty/sabre/dav/lib/DAV/Server.php",“line”:967,“function”:“getNodeForPath”,“class”:“OCA\DAV\Connector\Sabre\ObjectTree”,“type”:"->",“args”:[“INSTALL”]},{“file”:"/var/www/nextcloud/3rdparty/sabre/dav/lib/DAV/Server.php",“line”:1666,“function”:“getPropertiesIteratorForPath”,“class”:“Sabre\DAV\Server”,“type”:"->",“args”:[“INSTALL”,["{DAV:}getlastmodified","{DAV:}getetag","{DAV:}getcontenttype","{DAV:}resourcetype","{DAV:}quota-available-bytes","{DAV:}quota-used-bytes","{DAV:}creationdate","{http://owncloud.org/ns}permissions","{http://owncloud.org/ns}id","{http://owncloud.org/ns}fileid","{http://owncloud.org/ns}size","{http://owncloud.org/ns}favorite","{http://owncloud.org/ns}share-types","{http://owncloud.org/ns}owner-id","{http://owncloud.org/ns}owner-display-name","{http://owncloud.org/ns}comments-unread","{http://nextcloud.org/ns}is-encrypted","{http://nextcloud.org/ns}has-preview","{http://nextcloud.org/ns}mount-type","{http://nextcloud.org/ns}rich-workspace"],0]},{“file”:"/var/www/nextcloud/3rdparty/sabre/dav/lib/DAV/CorePlugin.php",“line”:355,“function”:“generateMultiStatus”,“class”:“Sabre\DAV\Server”,“type”:"->",“args”:[{“class”:“Generator”},false]},{“function”:“httpPropFind”,“class”:“Sabre\DAV\CorePlugin”,“type”:"->",“args”:[{“absoluteUrl”:“https://nextcloud:5444/remote.php/webdav/INSTALL","class”:“Sabre\HTTP\Request”},{“class”:“Sabre\HTTP\Response”}]},{“file”:"/var/www/nextcloud/3rdparty/sabre/event/lib/EventEmitterTrait.php",“line”:105,“function”:“call_user_func_array”,“args”:[[{“class”:“Sabre\DAV\CorePlugin”},“httpPropFind”],[{“absoluteUrl”:“https://nextcloud:5444/remote.php/webdav/INSTALL","class”:“Sabre\HTTP\Request”},{“class”:“Sabre\HTTP\Response”}]]},{“file”:"/var/www/nextcloud/3rdparty/sabre/dav/lib/DAV/Server.php",“line”:479,“function”:“emit”,“class”:“Sabre\Event\EventEmitter”,“type”:"->",“args”:[“method:PROPFIND”,[{“absoluteUrl”:“https://nextcloud:5444/remote.php/webdav/INSTALL","class”:“Sabre\HTTP\Request”},{“class”:“Sabre\HTTP\Response”}]]},{“file”:"/var/www/nextcloud/3rdparty/sabre/dav/lib/DAV/Server.php",“line”:254,“function”:“invokeMethod”,“class”:“Sabre\DAV\Server”,“type”:"->",“args”:[{“absoluteUrl”:“https://nextcloud:5444/remote.php/webdav/INSTALL","class”:“Sabre\HTTP\Request”},{“class”:“Sabre\HTTP\Response”}]},{“file”:"/var/www/nextcloud/apps/dav/appinfo/v1/webdav.php",“line”:82,“function”:“exec”,“class”:“Sabre\DAV\Server”,“type”:"->",“args”:[]},{“file”:"/var/www/nextcloud/remote.php",“line”:165,“args”:["/var/www/nextcloud/apps/dav/appinfo/v1/webdav.php"],“function”:“require_once”}],“File”:"/var/www/nextcloud/apps/dav/lib/Connector/Sabre/ObjectTree.php",“Line”:162,“Previous”:{“Exception”:“OCP\Files\StorageNotAvailableException”,“Message”:“Storage with mount id 1 is not available”,“Code”:1,“Trace”:[{“file”:"/var/www/nextcloud/lib/private/Files/Storage/Wrapper/Wrapper.php",“line”:527,“function”:“getAvailability”,“class”:“OC\Files\Storage\FailedStorage”,“type”:"->",“args”:[]},{“file”:"/var/www/nextcloud/lib/private/Files/Storage/Wrapper/Wrapper.php",“line”:527,“function”:“getAvailability”,“class”:“OC\Files\Storage\Wrapper\Wrapper”,“type”:"->",“args”:[]},{“file”:"/var/www/nextcloud/lib/private/Files/Storage/Wrapper/Availability.php",“line”:81,“function”:“getAvailability”,“class”:“OC\Files\Storage\Wrapper\Wrapper”,“type”:"->",“args”:[]},{“file”:"/var/www/nextcloud/lib/private/Files/Storage/Wrapper/Availability.php",“line”:92,“function”:“isAvailable”,“class”:“OC\Files\Storage\Wrapper\Availability”,“type”:"->",“args”:[]},{“file”:"/var/www/nextcloud/lib/private/Files/Storage/Wrapper/Availability.php",“line”:382,“function”:“checkAvailability”,“class”:“OC\Files\Storage\Wrapper\Availability”,“type”:"->",“args”:[]},{“file”:"/var/www/nextcloud/lib/private/Files/Storage/Wrapper/Wrapper.php",“line”:379,“function”:“hasUpdated”,“class”:“OC\Files\Storage\Wrapper\Availability”,“type”:"->",“args”:["",null]},{“file”:"/var/www/nextcloud/lib/private/Files/Cache/Watcher.php",“line”:127,“function”:“hasUpdated”,“class”:“OC\Files\Storage\Wrapper\Wrapper”,“type”:"->",“args”:["",null]},{“file”:"/var/www/nextcloud/lib/private/Files/View.php",“line”:1342,“function”:“needsUpdate”,“class”:“OC\Files\Cache\Watcher”,“type”:"->",“args”:["",{“class”:“OC\Files\Cache\CacheEntry”}]},{“file”:"/var/www/nextcloud/lib/private/Files/View.php",“line”:1384,“function”:“getCacheEntry”,“class”:“OC\Files\View”,“type”:"->",“args”:[{“cache”:null,“scanner”:{“class”:“OC\Files\Cache\Scanner”},“watcher”:null,“propagator”:null,“updater”:null,“class”:“OCA\Files_Trashbin\Storage”},"",“INSTALL”]},{“file”:"/var/www/nextcloud/apps/dav/lib/Connector/Sabre/ObjectTree.php",“line”:156,“function”:“getFileInfo”,“class”:“OC\Files\View”,“type”:"->",“args”:["/55D61FCD-423D-44BB-ACAF-8951EB377A98/files/INSTALL"]},{“file”:"/var/www/nextcloud/3rdparty/sabre/dav/lib/DAV/Server.php",“line”:967,“function”:“getNodeForPath”,“class”:“OCA\DAV\Connector\Sabre\ObjectTree”,“type”:"->",“args”:[“INSTALL”]},{“file”:"/var/www/nextcloud/3rdparty/sabre/dav/lib/DAV/Server.php",“line”:1666,“function”:“getPropertiesIteratorForPath”,“class”:“Sabre\DAV\Server”,“type”:"->",“args”:[“INSTALL”,["{DAV:}getlastmodified","{DAV:}getetag","{DAV:}getcontenttype","{DAV:}resourcetype","{DAV:}quota-available-bytes","{DAV:}quota-used-bytes","{DAV:}creationdate","{http://owncloud.org/ns}permissions","{http://owncloud.org/ns}id","{http://owncloud.org/ns}fileid","{http://owncloud.org/ns}size","{http://owncloud.org/ns}favorite","{http://owncloud.org/ns}share-types","{http://owncloud.org/ns}owner-id","{http://owncloud.org/ns}owner-display-name","{http://owncloud.org/ns}comments-unread","{http://nextcloud.org/ns}is-encrypted","{http://nextcloud.org/ns}has-preview","{http://nextcloud.org/ns}mount-type","{http://nextcloud.org/ns}rich-workspace"],0]},{“file”:"/var/www/nextcloud/3rdparty/sabre/dav/lib/DAV/CorePlugin.php",“line”:355,“function”:“generateMultiStatus”,“class”:“Sabre\DAV\Server”,“type”:"->",“args”:[{“class”:“Generator”},false]},{“function”:“httpPropFind”,“class”:“Sabre\DAV\CorePlugin”,“type”:"->",“args”:[{“absoluteUrl”:“https://nextcloud:5444/remote.php/webdav/INSTALL","class”:“Sabre\HTTP\Request”},{“class”:“Sabre\HTTP\Response”}]},{“file”:"/var/www/nextcloud/3rdparty/sabre/event/lib/EventEmitterTrait.php",“line”:105,“function”:“call_user_func_array”,“args”:[[{“class”:“Sabre\DAV\CorePlugin”},“httpPropFind”],[{“absoluteUrl”:“https://nextcloud:5444/remote.php/webdav/INSTALL","class”:“Sabre\HTTP\Request”},{“class”:“Sabre\HTTP\Response”}]]},{“file”:"/var/www/nextcloud/3rdparty/sabre/dav/lib/DAV/Server.php",“line”:479,“function”:“emit”,“class”:“Sabre\Event\EventEmitter”,“type”:"->",“args”:[“method:PROPFIND”,[{“absoluteUrl”:“https://nextcloud:5444/remote.php/webdav/INSTALL","class”:“Sabre\HTTP\Request”},{“class”:“Sabre\HTTP\Response”}]]},{“file”:"/var/www/nextcloud/3rdparty/sabre/dav/lib/DAV/Server.php",“line”:254,“function”:“invokeMethod”,“class”:“Sabre\DAV\Server”,“type”:"->",“args”:[{“absoluteUrl”:“https://nextcloud:5444/remote.php/webdav/INSTALL","class”:“Sabre\HTTP\Request”},{“class”:“Sabre\HTTP\Response”}]},{“file”:"/var/www/nextcloud/apps/dav/appinfo/v1/webdav.php",“line”:82,“function”:“exec”,“class”:“Sabre\DAV\Server”,“type”:"->",“args”:[]},{“file”:"/var/www/nextcloud/remote.php",“line”:165,“args”:["/var/www/nextcloud/apps/dav/appinfo/v1/webdav.php"],“function”:“require_once”}],“File”:"/var/www/nextcloud/lib/private/Files/Storage/FailedStorage.php",“Line”:210,“Hint”:“Storage 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is temporarily not available”}},“CustomMessage”:"–"},“userAgent”:“Mozilla/5.0 (iOS) Nextcloud-iOS/2.25.9”,“version”:“18.0.5.1”}

Does anyone has a hint how to Change the configuration to get the iOS app working?
Thank you in Advance.