supplied argument is not a valid ... class.t3lib_db.php on line 822
This error happened just after I installed Timtab weblog on TYPO3 4.2.1; and as I suspected, uninstalling Timtab resulted in the error going away.
Perhaps Timtab is not compatible with TYPO3 4.2.1 or something. I do notice that Timtab seems to be no longer maintained.
Anyway, I can live without Timtab, so removing the extension is not a big price to pay for not having to see the error message again.