WebDav Error 404 when searching file ios App

Can anyone point me in a direction where to search for in order to fix it? I’m really hitting rock bottom in terms of my skills…

When I use the (newest) iOS App to search a file the error 404 pops up. I can still upload download and edit files though. Following log is happening:

[LOG] Network response request: 2020-08-29 11:12:25 https:// remote.php/webdav, result: success(1216 bytes)
[LOG] Network request started: 2020-08-29 11:12:28 SEARCH https://
/remote.php/dav
[LOG] Network response request: 2020-08-29 11:12:28 https://*****/remote.php/dav, result: failure(Alamofire.AFError.responseValidationFailed(reason: Alamofire.AFError.ResponseValidationFailureReason.unacceptableStatusCode(code: 404)))

I have a basic snap Installation with as far as I know default settings.

Nextcloud v 19.01
Snap Nextcloud 19.01snap3
Snap version 2.45.3.1
Ubuntu 18.04

Today this was displayed in the logs:

Sabre\DAV\Exception\NotFound: Principal with name USERNAME not found

/snap/nextcloud/23027/htdocs/3rdparty/sabre/dav/lib/DAV/Tree.php - line 80:

Sabre\DAVACL\AbstractPrincipalCollection->getChild("USERNAME")

/snap/nextcloud/23027/htdocs/apps/dav/lib/Files/FileSearchBackend.php - line 153:

Sabre\DAV\Tree->getNodeForPath("files/Syste ... e")

/snap/nextcloud/23027/htdocs/apps/dav/lib/Files/LazySearchBackend.php - line 65:

OCA\DAV\Files\FileSearchBackend->search(SearchDAV\Qu ... }})

/snap/nextcloud/23027/htdocs/3rdparty/icewind/searchdav/src/DAV/SearchHandler.php - line 81:

OCA\DAV\Files\LazySearchBackend->search(SearchDAV\Qu ... }})

/snap/nextcloud/23027/htdocs/3rdparty/icewind/searchdav/src/DAV/SearchPlugin.php - line 123:

SearchDAV\DAV\SearchHandler->handleSearchRequest({ {DAV:}basi ... }}, Sabre\HTTP\Response {})

/snap/nextcloud/23027/htdocs/3rdparty/sabre/event/lib/WildcardEmitterTrait.php - line 89:

SearchDAV\DAV\SearchPlugin->searchHandler(Sabre\HTTP\Request {}, Sabre\HTTP\Response {})

/snap/nextcloud/23027/htdocs/3rdparty/sabre/dav/lib/DAV/Server.php - line 474:

Sabre\DAV\Server->emit("method:SEARCH", [ Sabre\HTTP ... }])

/snap/nextcloud/23027/htdocs/3rdparty/sabre/dav/lib/DAV/Server.php - line 251:

Sabre\DAV\Server->invokeMethod(Sabre\HTTP\Request {}, Sabre\HTTP\Response {})

/snap/nextcloud/23027/htdocs/3rdparty/sabre/dav/lib/DAV/Server.php - line 319:

Sabre\DAV\Server->start()

/snap/nextcloud/23027/htdocs/apps/dav/lib/Server.php - line 320:

Sabre\DAV\Server->exec()

/snap/nextcloud/23027/htdocs/apps/dav/appinfo/v2/remote.php - line 35:

OCA\DAV\Server->exec()

/snap/nextcloud/23027/htdocs/remote.php - line 167:

require_once("/snap/nextc ... p")