So I was previously using alexscaves-1.0.6 with citadel-2.4.7-1.20.1 in one of my worlds. Now after updating to alexscaves-1.0.8 with citadel-2.4.9-1.20.1 on forge-47.2.16, I crash in my worlds that loaded the older Alex's Caves. I tried deleting the old configs and it does nothing to help. Newly created worlds work just fine without an issue.
debug-1.log

Edit: Also alexscaves-1.0.6 with citadel-2.4.8-1.20.1 runs fine.
Edit 2: Huh, after running the version above, entering and quitting a world, and deleting the old configs again, alexscaves-1.0.8 finally worked for all my old worlds. Still going to leave this open just in case there's a need for investigation.