The walberla git submodule path is relative, but should be absolute.
This is an issue whenever HyTeG is cloned from somewhere else.
From https://i10git.cs.fau.de/hyteg/hyteg/-/blob/d27b73c59a3f8e0191eb7a4f0fc00ba40263964d/.gitmodules#L3:
url = ../../walberla/walberla