Shared folders became inaccessible after nextcloud upgrade

I moved from 22.2.7 to 22.2.10 to 23.0.8 to 24.0.4. After that the shared folders(!) appear to be inaccessible. The error messages in the log file say “Cannot decrypt this file, probably this is a shared file. Please ask the file owner to reshare the file with you.” Its not a shared file.

If I upload e new file to a shared folder, then it can be downloaded by others as usual. But the “upgraded” files cannot. The web browser shows a message “This site can’t be reached. ERR_INVALID_RESPONSE”.

Every helpful comment is highly appreciated

PS: The troubleshooting page (General troubleshooting — Nextcloud latest Administration Manual latest documentation) did not help.