Can not connect with any client

Some weeks ago, my syncronisation stops (no idea why).
So i decided to change to ownCloud 10.

Setup all worked properly and i can access via Browser. No problem.

But no client works. I installed the Desktop-Client (Windows) but i can not login via this client.

So i also tested to create a app-password as well. It also works not.

I tested also the WebDAV config in the browser. There comes the following message:
"This is the WebDAV interface. It can only be accessed by WebDAV clients such as the ownCloud desktop sync client."

I can also not connect with the andoid client.

Unfortunately there is no single solution to this as most issues causing these are related to various server misconfigurations. However there is a large FAQ available here at the forums showing all known information how to configure / fix your environment:

Of course i checked this.
I installed Nextcloud for testing....and it works fine....really strange

Which client version is that?
Anything useful in the server log files? owncloud.log and HTTP access.log and error.log?