The issue I have is, when I take the AR202 case MechJeb 2 and try to put it on my rocket, the new menu shows up to the left, but the part simply won't snap onto any rocket parts. Also, after trying to connect the part, no stock part will attach either. (If I don't use the MechJeb case at all, everything works fine) I can only get parts to connect again by making a new rocket. Finally, when this bug happens, the Launch button no longer works either.
Edit: KSP.log contains a lot of "MissingFieldException: Field '.EditorLogic.shipNameField' not found." and "NullReferenceException", both errors starting from when I tried to attach the part.
Kerbal Space Program - 0.20.2.186 (WindowsPlayer)
MechJeb 2.0.9.0

Can you post the error ? EditorLogic.shipNameField should be fixed in 2.0.9. You have no other mods ?

Yeah, I had KW and NovaPunch but an update to 0.21.1 fixed the issue. I guess it was a version mismatch.