Dear Kyle,
the problem turned out to be induced by a novelty introduced in the fresh 4.2.0 release, namely the optical photon generation by local sub-threshold energy deposition events, indicated in the release notes. This triggered the geometry tracking errors you experienced with optical photons. The respective bug fix will be included - together with others - in the upcoming minor release. Thanks a lot for your prompt report and sorry for the wasted time.
On a parallel note, your .err file displays at the top repeated messages questioning the soundness of the Sternheimer parameters you input for your Gd2O2S material. In fact, they yield a density effect correction with the wrong sign, which is then zeroed by FLUKA as calculating the stopping power. I’d advise to avoid overwriting those parameters as well as the average ionisation potential (just drop the respective STERNHEI and MAT-PROP cards).