Profile page problem with permalinks set to anything other than plain

Hello,

for some reason, the profile page of a logged-in member can not be accessed via the users account page unless the permalink settings are set to “plain”, as the “View profile” link in the user account (underneath the image) does some wrong re-writing as it seems.

Permalinks set to plain, link is like this: https://mydomain.com/?page_id=1353&lang=de&um_user=test
–> works fine

Permalinks set to anything else, link is like this: https://mydomain.com/de/benutzerprofil/test/
–> page not found

Any idea what is wrong and how to solve it?
Most other UM-Pages work fine as far as I can see when the Permalinks are not set to “plain”.

The setup is:
Wordpress Version 5.2.3
Ultimate Member Version 2.0.56
Polylang Version 2.6.5

Theme: twenty-nineteen

All other plugins habe been deactivated.

Thanks for any help.