I can just say that i only had this issue two times and it were both thief 1 missions. Every other mission, especially thief 2 missions run perfectly fine on the DarkLoader, for exampel the newest one"Matriarch" or some more complex ones like "The Violent End of Duncan Malveine", "Godbreaker", "Death´s Cold Embrace", "Looser", "Katharsis", the list goes on...

However, since theres no other explanation it has to be the DarkLoader. I wanted to try out the AngelLoader anyway: )

maybe the problem lies with modern thief 1 missions, since there aren´t many of them

yea i have. i have also played nearly every single mission without any issues, except one or two for thief 1, like this one. thats why i am asking. but mybe its time to switch the loader

yea, i know. Do you still use the DarkLoader or something else? I have the OG T1 and TGold installed, since this mission is adressed as a Thief The Dark Project Misson and both versions do not work, so maybe its the DarkLoader