Strange error after ungrade

I have the following error after I have upgraded from 2.65 to 2.80.  I copied over my old theme and made the changes in order to get it to work and it did for a few hours then suddenly this...

[A]System.Web.WebPages.Razor.Configuration.HostSection cannot be cast to [B]System.Web.WebPages.Razor.Configuration.HostSection. Type A originates from 'System.Web.WebPages.Razor, Version=1.0.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35' in the context 'Default' at location 'C:\Windows\Microsoft.Net\assembly\GAC_MSIL\System.Web.WebPages.Razor\v4.0_1.0.0.0__31bf3856ad364e35\System.Web.WebPages.Razor.dll'. Type B originates from 'System.Web.WebPages.Razor, Version=2.0.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35' in the context 'Default' at location 'C:\Windows\Microsoft.Net\assembly\GAC_MSIL\System.Web.WebPages.Razor\v4.0_2.0.0.0__31bf3856ad364e35\System.Web.WebPages.Razor.dll'.

Everything is fine when i revert to the default theme but i cant find the solution to the prob? Any ideas?

S

Hi

You reference to a wrong version of 'System.Web.WebPages.Razor' in your project. It must be v4.0_2.0.0.0. Make sure your project web.config and project References are refering to same version.

thanks for the feedback, turns out i had copied a webconfig file into a plugin folder my mistake...

I got this after upgrading and copying my "custom" theme folder from version 2.65 to version 2.80.   I deleted the folder and it worked again.  I assume there is a file in there that references the old version of System.Web.Webpages.razor.  It is running again, but my theme is gone .   :-)   and :-(