Ah, so it doesn't look like it was the previous version of Procedural Parts that was causing the odd GUI issue/debug errors. Even with the updated Procedural Parts that uses KAE 1.7.5, the issue still persists. However, removing the KAE dll fom RealFuels or otherwise disabling the mod still resolves this issue like before. I was also wrong when I referred to the "updated" KAE dll in RealFuels, as it turns out it still uses 1.7.3 (what I found in the directory), so at this point I'm completely lost. All I can say at this point is that, on my end (and at least one other person), having Real Fuels 10.4.1 and PP 1.1.4 present will break functionality of the latter, with the error message: [Error]: ItemPrefab for control type 'UI_ChooseOption' not found.