The problem most likely is that OBS-MP and the game are not running at the identical administrative level. For example, if the game is running as admin, OBS-MP has to be running as admin, or if the game is running as non-admin, OBS-MP also has to be running as non-admin, otherwise game capture won't hook properly. This only applies to OBS-MP, and I'm going to fix it at some point.
Here is what happened: in OBS-MP, I unwisely thought that I could just always use the anti-cheat compatibility hooking method to hook game capture to the game, and I mistakenly that it would act identically to the normal hooking method (which creates a remote thread). Unfortunately, it introduced this administrative-level issue as you can see, so I need to switch it back to using the older more reliable method and make anti-cheat compatibility an option instead.