Android 10
Nexcloud 20.0.8
Joplin App 1.7.5
I've connected 2 macs with joplin to my nextcloud already and it works fine. But the joplin app on android failed to log in. Am I doing something wrong or is there a bug? Tried the following URL's:
https://nextcloud.domain.de/remote.php/dav/files/username/joplinfolder/
Message: Not a valid URL
(this is how it works on the macs)
https://nextcloud.domain.de/remote.php/dav/files/username/
Message: Not a valid URL
https://nextcloud.domain.de
Message: PROPFIND: Unknown error 2(405)
THANK YOU!! 
Nextcloud 20.0.8
Joplin 1.7.11
Windows 10
Android 9
LineageOS 17.1 (Android 10)
Linux Mint 20.1
On all devices I have used the the Webdav connector not the Nextcloud one, and a path in the format:
https://nextcloud.domain.de/remote.php/webdav/joplinfolder
Also you should double check your URL because the message "not a valid URL" really means what it means, that it's generally not valid (not just as a Nextcloud URL). Perhaps there's an extra space or something in it.
Thank you so much, @dpoulton that works! 
(But I wonder why there is a nexcloud function, if it's not working? 
@laurent
You're right. This happens very often. But this was copy-paste from an already working device.
Possibly because Nextcloud changes things quite regularly and they are under no obligation to tell Joplin what changes they have made!!