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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
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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): 16.0.4
Operating system and version (eg, Ubuntu 17.04): Ubuntu Linux 18.04.3
The issue you are facing: My folder refuses to sync. Keep getting the error displayed in the log below.
Is this the first time you’ve seen this error? (Y/N): Yes. Al though previously it would never sync automatically, I had to go in and expand the remote folder to make it re-scan.
Steps to replicate it:
- It really just happens by itself, whenever it tries to sync.
The output of your Nextcloud log in Admin > Logging:
Fatal webdav Sabre\DAV\Exception\ServiceUnavailable: TypeError: Argument 2 passed to OCA\DAV\Connector\Sabre\Directory::__construct() must implement interface OCP\Files\FileInfo, bool given, called in /var/www/nextcloud/apps/dav/lib/Upload/UploadHome.php on line 98
<<closure>>
{closure}("*** sensiti ... *")
/var/www/nextcloud/3rdparty/sabre/event/lib/EventEmitterTrait.php - line 105:
call_user_func_array(Closure {}, [ "*** sensi ... "])
/var/www/nextcloud/3rdparty/sabre/dav/lib/DAV/Server.php - line 466:
Sabre\Event\EventEmitter->emit("beforeMethod", [ "*** sensi ... "])
/var/www/nextcloud/3rdparty/sabre/dav/lib/DAV/Server.php - line 254:
Sabre\DAV\Server->invokeMethod("*** sensiti ... *", "*** sensiti ... *")
/var/www/nextcloud/remote.php - line 72:
Sabre\DAV\Server->exec()
/var/www/nextcloud/remote.php - line 168:
handleException(TypeError {})