SOLVED:
I had downloaded another simulation software on my pc which downloaded a MS MPI package that was conflicting with the one used by blueCFD. This was causing the parallel run to fail…
For those who don’t know what I’m talking about, to put it simply, blueCFD uses the msmpi to enable parallel processing on windows. If there are conflicting versions on your machine, the parallel runs might not work. A good indicator that this is the case would be that running processes in series (without decomposeParDict) would work, but parallel (with decomposeParDict) runs would fail. This applies to any process be it SnappyHexMesh or the Solution solver.
Following step 1 here remedied the problem for me.