I copied the file tigervnc to /etc/pam.d, restarted xvnc.socket. But no luck. The file tigervnc does indeed exist in that directory in an an older snapshot of tumbleweed (where xvnc works fine).
I’m still wondering what is meant by “Until: Sat 2023-02-18 08:46:18 CET; 2min 32s ago” (see my first post above).