2
One or more comments are connected to a module that either has never existed or has been uninstalled.
How this happened, I can't really say, but if you can, then load up the xoopscomments table in a tool like phpmyadmin and note down the com_modid for the 10-20 latest comments (highest com_id)
Then look in the modules table and see if any of the noted com_modid's are not present in the modules table's mid column. If you find one (or more) delete the corresponding rows in the xoopscomments table.
"When you can flatten entire cities at a whim, a tendency towards quiet reflection and seeing-things-from-the-other-fellow's-point-of-view is seldom necessary."
Cusix Software