Custom Simulink code fails to compile moving from 2014a to 2016a

Hi, my custom code integration working in Simulink 2014a does not work anymore with the same settings and same compiler in 2016a. What I did:

I used Simulink Coder to generate C Code from a Simulink Model and ran it with coder.ceval in another Simulink System inside a Matlab Function Block (various reasons to do so, working perfectly).

I included the model, model_data, and my own "caller function" .h and .c in the Simulink options (twice at both possible options). Ìnside my caller function, I #Included the model.h and model_data.h again (it was not working without this). Moving from 2014a to 2016a though, it does not work anymore. Although I coded the Simulink model again with the 2016a Coder version to make sure all Matlab types are correct for Version 2016, the exact same implementation and settings do not work anymore. The only further hint I have is:

• Compilation Error: "test_ceval_simulation_sfun.c c:\program files\matlab\r2016a\extern\include\tmwtypes.h(788): fatal error C1083: File (Include) cannot be opened: "stddef.h": No such file or directory"

What do you guys think? Did you ever successfully did "model.slx -> c code -> run in another model" in 2016a?

Thanks for your answers in advance!