I re-installed XOOPS 2.2.3 but I kept the old database [i.e. re-uploaded XOOPS files, didn't delete database and went through installation process again].
This allowed me to access my admin area although the error message still existed on the frontpage.
I then updated the System module, uninstalled/installed the Extended Profiles module, and uninstalled unneeded modules.
It seems to be working now. I've a small amount of duplicated info in my database but I can live with that. Also, sometimes I get errors when trying specific tasks with modules, but I just reinstall/update those modules and it works fine.
Cool.