Aug 18, 2010 08:26 AM|TylerMan|LINK
An FYI for others with the same problem - I had this error thrown for every control on my web page each time it was used and then one other error - an AJAX control was not recognized. The dll was already in the references but the tagPrefix was missing
from the web.config. I got that line in the web.config and all errors went away. Problem with that one control escalated to problems with every other.
Hope this helps;