I just upgraded a local version of my xoops site (from v.2.4.5 to 2.5.5).
I faced 2 main issues:
1. the content of all custom blocks is not displayed (only the title of the block is shown). I created new HTML and PHP blocks, I updated the system module, I deleted the cache (both browser and xoops) and I checked the access rights, but the problem still remain.
2. RSS feed generation doesn't work. The [xoops_site]/backend.php page returns the following message (my English translation):
Error during xml reading: no element found
Line number 1, column 1: