First of all, thank you for your tipps and help.
I dont really find the "Use extended permissions" button in the RPHook options. Not in general, plugins, advanced settings and game settings.
I'm actually not sure whether you want to troll me or just didnt read why I wrote. The last point you suggested was something I didnt try before, it didnt have any effect on my problem after I tried though. The other things you mentioned were things I already tried, as stated in my original post (such as creating an exception for the whole gta folder, deactivating my AV (I use Windows Defender so no third party AV programm), trying to start RPHook as admin, installing LSPDFR via manual install).
I read under another post the following:
"Please make sure that you are running Rage from the original Grand Theft Auto V folder.
1. You can not move your Grand Theft Auto V folder.
2. You can not rename your Grand Theft Auto V folder.
3. You can create a backup folder in the same path and switch names to play Online & Modded."
As mentioned before I have my original GTA5 located on my C:\. My copy with mods etc. is on D:\. If obviously that is the reason why I have these issues then ok, but when I last tried on monday it suddenly worked after I changed the properties so I dont start RPHook in admin mode. After I installed several other mods (like EUP) the problem suddenly occured AGAIN.