WebDAV error on external url - but everything seems fine

Hi,

on the …/admin/overview page I get the following error:

Your web server is not yet set up properly to allow file synchronization because the WebDAV interface seems to be broken.

This error only shows up when I am using the external url (from the internet).
When I use the internal url I do not get any errors.

The strange thing is, that everything works fine. There are no errors in the logfiles.
I can access the …/remote.php/webdav url without errors.
I am able to upload and download files through the external url i.e. with the desktop client.

So the only “problem” is the error message itself.

Has anybody an idea where this coud come from or if there might other limitations that I am not aware of right now?
Where should I search for more information?

Many thanks,
Stefan

Nextcloud version: 17.1