I recently upgraded from 5.4 to 8.0 and am getting the following error message:
System.ArgumentException: Type "Telerik.Sitefinity.Services.Comments.Impl.CommentService, Telerik.Sitefinity.Services.Comments.Impl" cannot be resolved.
I would make sure that the assembly is referenced in the Visual Studio SitefinityWebApp project and ensure that it is the 8.0 version of the assembly.
Also, you may want to compare your solution to a blank copy of the solution files to see what could be missing. I generally use a tool called Beyond Compare from Scooter Software to compare these files. Hope this helps.
I was using WinMerge and for some reason it did not catch this DLL. I added it to the solution and it did resolve the issue.
Thanks for the response Craig!