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Take a look at the callstack to see where the error originates from?
My guess is that you have some third party code (addon or similar) that is not upgraded to a CMS8 version. That error is typically something you could see if a code compiled against some 7.x.x version of EPiServer is executed in a 8.x.x. environment of EPiServer.
Everything was working perfectly then I rebuilt my solution and now when I'm trying to save, I get the following error:
Method not found: 'EPiServer.Core.ILanguageSelector EPiServer.Core.LanguageSelector.AutoDetect(Boolean)'.
Any ideas? I'm so confused why a simple change all of a sudden is causing such a mess.
I've reverted the one change I made, but with no luck. Could this be a database issue all of a sudden?
I'm on EPiServer 8.3.0.