Thanks for your help again!
I followed your recommendations and installed TBB (FYI the wget link for oneTBB in the doc didnt work for me at first, had to pass the version number directly) , reinstalled RcppParallel, StanHeaders and rstan.
The Flags check out:
-L'/home/julian/oneapi-tbb-2021.1.1/lib/intel64/gcc4.8' -Wl,-rpath,'/home/julian/oneapi-tbb-2021.1.1/lib/intel64/gcc4.8' -ltbb -ltbbmalloc>
and after also removing tbb from StanHeaders it works like a charm now!!
Many thanks!!
also tagging @paul.buerkner to let you know that this indeed works now :)