Created Unassigned: Error trying to add extender [27904]

While trying to add extender, get this error:

Unable to reference extender assembly AjaxControlToolkit, Version=15.1.2.0, Culture=neutral, PublicKeyToken=28f01b0e84b6d53e.

I looked at the dll in the bin directory, and it is the correct version (15.1.2.0).
.net version is 4.5.2.

Help is most appreciated - this is a show stopper!

Saavik