I've found my problem. If anyone got the same issue (version 136-139 not showing up the windows ingame) you should take a look in the Kerbal Space Program\KSP_Data\output_log.txt. My problem was a AntiMalware tool blocking the .dll This is out of the Output_log: Failed to load assembly D:\...\SteamLibrary\SteamApps\common\Kerbal Space Program\GameData\MechJeb2\Plugins\MechJeb2.dll: System.UnauthorizedAccessException: Access to the path "D:\...\SteamLibrary\SteamApps\common\Kerbal Space Program\GameData\MechJeb2\Plugins\MechJeb2.dll" is denied. at System.IO.FileStream..ctor (System.String path, FileMode mode, FileAccess access, FileShare share, Int32 bufferSize, Boolean anonymous, FileOptions options) [0x00000] in <filename unknown>:0 at System.IO.FileStream..ctor (System.String path, FileMode mode) [0x00000] in <filename unknown>:0 at AssemblyLoader.LoadExternalAssembly (System.String file) [0x00000] in <filename unknown>:0