I will try to extend this post with a reproducible example later, but would like to warn everyone who has not installed the 18.0.1 compiler update yet. Submodules seemed to work just fine with 18.0.0, but lead to tons of problems (with valid code) when trying to compile and link code using 18.0.1.
it seems, I have run into the same issue:
I filed a ticket at OSD. It's really a pitty. 18.0 produces an ICE and 18.0.1 cannot handle submodules correctly.