Catchable fatal error

Hi

I have a problem with Catchable fatal error: Object of class __PHP_Incomplete_Class could not be converted to string in /home/johanhor/public_html/spiritpathwaysfoundation.com/wp-includes/functions.php on line 1463

All seems to work but people cannot login, the password seems to be a problem. When I go in at the backend and change the password and click on update the error occurs.

I just upgraded to 3.0.1 and made my site Multi Site. Your suggestions will really help. There are other forum topics on this but they seem to be technical.