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Sorry to hear you’re facing problems 
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In order to help you as quickly as possible, before clicking Create Topic please provide as much of the below as you can. Feel free to use a pastebin service for logs, otherwise either indent short log examples with four spaces:
example
Or for longer, use three backticks above and below the code snippet:
longer
example
here
Some or all of the below information will be requested if it isn’t supplied; for fastest response please provide as much as you can 
Nextcloud version (eg, 12.0.2): 15.02
Operating system and version (eg, Ubuntu 17.04): Ubuntu 16.04
Apache or nginx version (eg, Apache 2.4.25): Apache 2.4.18
PHP version (eg, 7.1): 7.1.25
The issue you are facing:
Fatal webdav Messages :
Sabre\DAV\Exception\ServiceUnavailable: Could not open file
Is this the first time you’ve seen this error? (Y/N): Y
Steps to replicate it:
The output of your Nextcloud log in Admin > Logging:
Fatal webdav Sabre\DAV\Exception\ServiceUnavailable: Could not open file
/var/www/html/nextcloud/3rdparty/sabre/dav/lib/DAV/CorePlugin.php - line 85:
OCA\DAV\Connector\Sabre\File->get()
<<closure>>
Sabre\DAV\CorePlugin->httpGet(Sabre\HTTP\R ... "}, Sabre\HTTP\Response {})
/var/www/html/nextcloud/3rdparty/sabre/event/lib/EventEmitterTrait.php - line 105:
call_user_func_array([ Sabre\DAV\ ... "], [ Sabre\HTTP ... }])
/var/www/html/nextcloud/3rdparty/sabre/dav/lib/DAV/Server.php - line 479:
Sabre\Event\EventEmitter->emit("method:GET", [ Sabre\HTTP ... }])
/var/www/html/nextcloud/3rdparty/sabre/dav/lib/DAV/Server.php - line 254:
Sabre\DAV\Server->invokeMethod(Sabre\HTTP\R ... "}, Sabre\HTTP\Response {})
/var/www/html/nextcloud/apps/dav/lib/Server.php - line 301:
Sabre\DAV\Server->exec()
/var/www/html/nextcloud/apps/dav/appinfo/v2/remote.php - line 35:
OCA\DAV\Server->exec()
/var/www/html/nextcloud/remote.php - line 163:
require_once("/var/www/ht ... p")
The output of your config.php file in /path/to/nextcloud (make sure you remove any identifiable information!):
PASTE HERE
The output of your Apache/nginx/system log in /var/log/____:
PASTE HERE
