I recently had my website upgraded to SQL 5.1.63 and PhP 5.3.15. I am using Joomla 2.5.4. The ISP that did the upgrade messed up and SQL has now been recompiled to properly include all the necessary PhP extensions about 6 times. I thought things were fine until I tried adding tags to new articles - they won't add - they just vanish. And none of my other articles had their tags either and I cannot re-add them. Then I realized that I was unable to add new users to a K2 group. So while my articles are still grouping and presenting themselves ok, I have lost the ability to have tags, groups and, by the way, K2 comments.
Anyone with any clues on how to fix this?

Hi. Go to Joomla! configuration and enable system debugging. After that try to add a tag or a user to a K2 group. If there is an SQL error the debugger will show it to you.