@rfc2822
I checked all my characters in my password for ISO-8859-1 characters and all of them are in this list.
-> No, I don’t use non-ISO-8859-1 characters.
I think (I’m not 100% sure) that I don’t use ldap authentication.
If you show me how I can test it, I will do it 
I can’t directly access webdav in my browser. When I go to “https:MYDOMAIN/CLOUDFOLDER/remote.php/dav/” I get the following message:
“This is the WebDAV interface. It can only be accessed by WebDAV clients such as the ownCloud desktop sync client.”
This happens when I’m logged into Nextcloud on my browser (in another TAB), but if I’m NOT logged into nextcloud, the site shown above forces myself to login. After I entered my username and password the site loads a while and then asks again for my login details. This happens as long as I repeatedly enter my username and password. If I abort the login I get the message “Unauthorized” with some technical details.
And I don’t know how to use curl. I installed it, but I can’t really work with it, because I don’t understand how it works.
If you show me a command I how I can use it, I will test it!
I also can create a testuser for you and send you the password if you want.
EDIT:
I tried it with a much weaker password and - what a wonder - it seems to work fine…
Thanks so much for your help!