So, I fixed it. It seemed to be a problem with something inside the mod folder, because as soon as I opened up OpenIV and went into the mod folder I got a messaged that I have and outdated update.rpf file. I clicked on replace & update and everything is running fine again.
Might be caused by installing the LSPD vehicle pack, because I reinstalled this a few mins ago, after everything worked, (because when I patched the update.rpf file OpenIV told me all changes which I made before inside the folder are gone now) but then my game started crashing when I started GTA, so I removed this pack and everything works again.
Might be the cause for the first issue, might also be a second one, who knows.
All in all, thank you for your help and have a good day everyone.