Thank you Joris,
It seems that you were right. This solved the problem. I guess because the test bench template writer also made the package that Modelsim didn't cry about not finding a type.
Now that the original package is included Modelsim started telling me that it doesn't know which constants to take. For example work.gen_mux_data_type.m or work.my_data_types.m. I solved this problem by throwing gen_mux_data_type out of the test bench.
Now Modelsim doesn't give errors anymore and just runs the test bench (which isn't complete).
Conclusion is that I learned never to use pre-made packages in test benches and just to include the original. Thank you for helping me learn this.
Greets, Jan