Just to add: I just had this exception (yes, i used visual webdeveloper 2010 in 2025, i probably shouldn't...). But i had it when the aspx-page loaded, so the yellow message gets displayed in the browser window. my problem was that i changed the sql query of the sqldatasource and didn't adjust the columns in the gridview... so, somehow it couldn't say that columns weren't matching or sth, it just displayed this confusing message, that made me feel like i missed to add a dll or i referenced the wrong version of some dll. anyway, after i imported the new columns in design view, everything worked again.