I had the same problem; here's what the issue was: my application with the App_Themes
folder was in d:\work\MyApp20\MyApp\MyApp
; this was because I also added 3 more solutions (projects) to my application; mapped as "Virtual Directory" was only "d:\work\MyApp20\MyApp\
", not the full path.
Resolution: remove the existing virtual directory and add a new one for "d:\work\MyApp20\MyApp\MyApp
"; this way I didn't get the error message. Also, don't forget to set the version of ASP.Net (I had 2 versions, so the 2.0 was required in this case)
Ghiutzu Pop, firstname.lastname@example.org