I'm having the same exact issue. I've narrowed it down to only xleveling/xskills. I run my server on G-Portal, and even with the server on 1.15.1, and the mod versions of 1.15.1 I get this crash. I received the crash on the 1.15.0 version as well, with the pre 1.15.1 version of the mods. So something changed when updating to .1.15.0+ that is causing this issue. Which is a real shame because I really enjoy this mod.