Hi, @Sven H
Thank you for posting in Microsoft Q&A forum.
According the documentation, before we upgrade the OS of SCCM site systems, we need to remove the WSUS role from the server if it's installed. You may keep the SUSDB and reattach it once WSUS is reinstalled.
For now, you may try to remove and reinstall WSUS role to see if it helps.
For the details:
https://learn.microsoft.com/en-us/mem/configmgr/core/servers/manage/upgrade-on-premises-infrastructure#before-upgrade
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