I upgraded my Xoops-systems from version 2.5.0 to 2.5.5 Final and then my Tooltips in the Admin did not work anylonger. It was a strange problem, but I found the solution. I uses the theme "default" in the Admin and when I upload the new file /public_html/modules/system/themes/default/js/tooltip.js the script don't work. When I use the old script, everything works Ok. I don't see any differences between those files, so I don't know what's the reason of the problem
I post it here, maybe someone has the same problem...