Yesterday I've updated this blog website (and one other) from Serendipity version 2.1.5 to Serendipity version 2.3.1 and man-oh-man, was I glad I did have an older backup of the environment somewhere! After updating several issues arose from Java errors to plan 403 and 500 messages.. I tried several solutions, like applying the update again, changing PHP versions and settings, but nothing helped. I ended up with a backup made in June this year with Serendipity version 2.1.4, copying the update 2.3.1 to that location and running the update again. Now it ran smooth, this article is the proof
This shows, once again, the importance of a backup before upgrading or updating!