I already solved this, just thought I would share.
I ran into this exception while upgrading a project to v3. This was a runtime exception, and not a compile one.
|System.IO.FileLoadException: Could not load file or assembly 'SD.LLBLGen.Pro.ORMSupportClasses.NET20, Version=18.104.22.168, Culture=neutral, PublicKeyToken=ca73b74ba4e3ff27' or one of its dependencies. The located assembly's manifest definition does not match the assembly reference. (Exception from HRESULT: 0x80131040) C:\xx\LicenseeManager.cs(116): at System.Boolean GeniusCode.AppSupport.BL.Managers.LicenseeManager+Validations.ValidateAdminKey(SD.LLBLGen.Pro.ORMSupportClasses.IDataAccessAdapter adapter, System.String accessKey, System.Guid adminKey, System.Guid& licenseeId, System.String& licenseeName)
I thoroughly checked all my projects but all references pointed to 3.0. I uninstalled 2.6, regenerated (from scratch) my DAL, and tried a few other things, but kept getting the exception at runtime.
I solved it and blogged about it here.
The problem was somehow my executable project was depending on a DAL that depended on 2.6, even though my actual DAL depended on 3.0
Thankfully, removing and re-adding the reference to my DAL fixed the problem.
Wasted 2 days on this one.