OK folks, now i'm really angry, not to say pissed.
After getting this error with 5.4.0 i made a total rollback (haha, i'm having a fullbackup, but anyhow.....), i waited for 5.4.1 and made again an upgrade today.
Guess what? New behaviour but still not the desired one.
That means that the old rule is still valid: What's new in the new version? New bugs...
When clicking on the user name the installation just reloads the actual page (different, in 5.4.0 it was jumping to homepage). When going directly to the page where the userprofile module is placed it just tells me that "This module is only intended for use on a page which is defined as the profile page for the portal"
Yes, this specific page is set as profile page in the site settings and i tested the names "User Profile"; "Profile Page" and "Banana", always the same.
Yes, i restarted IIS, i killed cache and cookies.....
OK, as i need a working (!!!!) profile page i will again roll back to 5.3.1