There was a change to the UE API I wasn't aware of, which led to this problem happening.I've uploaded a new version, so please download "HammUEr 1.8 for 4.19 fixed" and try again.
Oh thank you very much, i was already so desperate, that i completely deleted, redownloaded and reinstalled ue4 and csgo :D
Edit: Yes, with the new version it creates materials-instances and decals as expected.
