I am not sure if this is a Boinc problem or a Linux/QT problem. I suspect the latter, but who knows? Cross-posted on openSUSE mailing list.
OS: Linux openSUSE 15.6
Kernel version: 6.4.0
QT version: 5.15.12
BOINC-manager command line: kdesu -u boinc /usr/bin/boincmgr

Boinc is running as a systemd service, username boinc. That commandline has been working properly until minutes ago, after a kernel update and reboot. Now, I get the following error in the system log:

2024-10-09T10:39:04.596734-06:00 hadron systemd[15459]: Started BOINC Manager.
2024-10-09T10:39:04.675709-06:00 hadron kdesu[21534]: Qt: Session management error: Authentication Rejected, reason : MIT-MAGIC-COOKIE-1 authentication rejected
2024-10-09T10:39:11.400889-06:00 hadron su[21576]: (to boinc) hadron on pts/6
2024-10-09T10:39:11.402261-06:00 hadron su[21576]: pam_unix(su:session): session opened for user boinc by (uid=1000)
2024-10-09T10:39:11.404816-06:00 hadron su[21576]: pam_unix(su:session): session closed for user boinc
2024-10-09T10:39:11.520236-06:00 hadron su[21579]: (to boinc) hadron on pts/6
2024-10-09T10:39:11.521441-06:00 hadron su[21579]: pam_unix(su:session): session opened for user boinc by (uid=1000)
2024-10-09T10:39:11.550926-06:00 hadron su[21579]: pam_unix(su:session): session closed for user boinc

In reply to hadron's message of 9 Oct 2024:

I am not sure if this is a Boinc problem or a Linux/QT problem. I suspect the latter, but who knows? Cross-posted on openSUSE mailing list.

Problem solved (hopefully permanently).

It has to have been a problem early in the system startup routines. I tried rebooting a second time, and now the gui manager is running fine.