You had a conflict between episerver.dll 4.61.0.83 and 5.1.281). 'c:\WINDOWS\assembly\GAC\EPiServer\4.61.0.83__8fe83dea738b45b7\EPiServer.dll' and 'c:\WINDOWS\assembly\GAC_MSIL\EPiServer\5.1.281.0__8fe83dea738b45b7\EPiServer.dll' During installation of the MSI-package EPiServer 5.1.281.0 is registered in the GAC. Your 4.61-site does unfortunately seem to have problem separating the two dll:s (or rather the .NET-environment). The easiest solution seems to be to remove version 5.1.281.0 from the GAC. This would at least solve your problem. The reason for the conflict is interesting but we should probably wait until RC1 to dig into this kind of problem. Version 281 (the CTP) has some known issues when it is running on the same machine as 4.6x so it is probably a waste of (your) time to track this problem down. I hope a removal of the EPiServer.dll (5.1.281.0) from the GAC will solve your problem.

I got the exact same error message after trying to run a 4.61 site having EPiServer CMS RC2 installed. First I removed CMS dlls from the GAC. Then when I then tried to run my 4.61 site I discovered that the web.config file had been changed. A section containing CMS assembly bindings had been added to the compilation section. By removing this section I got my 4.61 site up and running again (I also uninstalled RC2, but not sure if that was necessary). Note that the issue with web.config being updated also happens to a colleague of mine (he's running version 5.1.303.0).

I had the same problem. web.config for my 4.61 installation was changed. In section system.web/compilation/assemblies a number of references to CMS 5 assemblies were added. Since multiple episerver.dll's were referenced a conflict occured.

Solution: Remove the extra lines from web.config. That did the trick for me. I did not have to remove any dll's from the GAC.