These are invariably due to OTHER dlls which aren't loading into KSP properly.
Check your KSP.log for lines from "[Compatibility Checker]" and see if it mentions incompatible mods. Try updating everything using CKAN, or just pave your KSP install and reinstall all the mods from scratch (you can copy your old save file(s) to the new install).
The kinds of errors in the logs to look for should look like this:
[LOG 22:23:21.415] [MechJeb2] Loading Mechjeb 2.14.1
[ERR 22:23:21.421] MechJeb caught a ReflectionTypeLoadException. Those DLL are not built for this KSP version:"
[ERR 22:20:06.295] FMRS 1.0.1.0 GameData\FMRS\FMRS.dll
That is an example of a busted FMRS mod causing MechJeb windowing to fail. Whatever mod(s) are listed there will be the culprit.
These are invariably due to OTHER dlls which aren't loading into KSP properly.
Check your KSP.log for lines from "[Compatibility Checker]" and see if it mentions incompatible mods. Try updating everything using CKAN, or just pave your KSP install and reinstall all the mods from scratch (you can copy your old save file(s) to the new install).
The kinds of errors in the logs to look for should look like this:
That is an example of a busted FMRS mod causing MechJeb windowing to fail. Whatever mod(s) are listed there will be the culprit.