Is anyone having the same issues?
Make sure you install the mods in right folder: $KSP_Install_Dir / game / GameData /
$Directory_Where_Mod_Recides. In Vanilla game, the Folder "Squad" recides in GameData and these are official parts. You put every mod directory next to "Squad".
Make sure to install all mods according to instructions. Typically every mod resides in own directory. Do not rename this directories.
Make sure the mod "ModuleManager" is of latest version and installed. Its a mod consisting of a single DLL file
(regardless of OS), which must be put directly into GameData. This "mod" is vital, because its ModuleManager, who does patching the files. Yes, you need this mod to use any mods.
Make sure that ModuleManager DLL is ONLY present in GameData - and nowhere else inside GameData, especially not in any mod subdirectories. Sometimes the modders package it along, that causes game to execute ModuleManagers of different versions twice (or more). Usually this causes EXTREME loading time.
Highly recommend KSP-AVC mod, because it will check all mods for updates, list them and list any conflicts. That mod will also inform you of incompatibilities or why any mod is not loaded.