It is now public because I reviewed the flag that was raised by the spam classification and rejected the classification as a false positive, restoring the post to public visibility.
I mention this fact only to prevent confusion for those who will now visit the link and say "works fine for me". I don't know whether this error message is the expected behavior of the system during the time when the post was hidden due to the spam classification. It seems reasonable to me (only showing the notification shared by @sterretje to those who were already viewing the topic at the time it was hidden).