One of my clients had this issue – unable to update some of the wordpress users. When trying to update some of the user, he was getting this error
Catchable fatal error: Object of class __PHP_Incomplete_Class could not be converted to string in C:\web\wp28\wp\wp-includes\functions.php on line 1463
I went through many fixes, suggested in different threads, to edit the wordpress core file wp-includes\functions.php. None of them worked.
Since it only affects certain users I figured out that, the error may be because of the details of that particular user. I checked the database. I have seen some user meta keys related to a plugin which is uninstalled improperly. I deleted those meta keys and the error has gone away…