Extracting a nightly build over RC2 is a CML condition, never do that.
Well, it's too late now anyway, so the best thing you can probably do is delete the share folder, all dlls, and the application and unpack the nightly build again. If that fails again, remove the Code::Blocks folder as a whole and start over.
The error message probably comes from a plugin that fails to load since it links against the old version's dll (if nothing else is wrong). Be aware that a lot nastier things can happen too, though (for example unexpected crashes because resources are invalid). So, really... never mix up different versions.
There is unfortunately no way to convert RC2 settings to later releases. Not only do they use a different storage system (this would be the least problem), but the structure and naming is widely different, too.