I get this quite often when moving webparts to other servers. In addition to the usual troubleshooting provided in posts like these:
One thing I have found I need to do is check that the web.config file has all of the appSettings keys I am using in my code. If these are missing it can throw this error.
Another thing to check is that the web.config “” is set to “Full” if your dll requires that.
Additionally, make sure any files you need in the “bin” folder are there if you are using that instead of the GAC for anything.