Nextcloud version (eg, 10.0.2): 12.0.0.
Operating system and version (eg, Ubuntu 16.04): Debian 8
Apache or nginx version (eg, Apache 2.4.25): 2.4.10
PHP version (eg, 5.6): 5.6.30
Is this the first time you’ve seen this error?: Yes
Can you reliably replicate it? (If so, please outline steps):
If you mount an external drive over WEBDAVS with encryption you cannot access this mount point.
Message is "Speicher ist vorübergehend nicht verfügbar"
Without encryption everthing works fine!
The issue you are facing:
The output of your Nextcloud log in Admin > Logging: no log entries
The output of your config.php file in /path/to/nextcloud (make sure you remove any identifiable information!):
The output of your Apache/nginx/system log in /var/log/____: no log entries
Thx for support
Remember, this information may be requested if it isn’t supplied; for fastest response please provide as much as you can 
Feel free to use a pastebin service, otherwise log files can be indented with 4 spaces on each line to present them in a friendlier way on the forum.