Both, Sponge and the enhanced third-party transition code, re-implement the PlayerList.transferPlayerToDimension method, as such they're naturally not compatible.
Luckily Sponge wins, so BP just falls back to non-enhanced transfers and everything is fine (but not fancy).
Not yet sure how to fix this. We'll probably have to inject into the sponge code at a very specific location and take it from there (be sure to respect sponge events!).

your fix seemed to work, both with single player and on sponge server i was able to jump between worlds 5+ times in a row mid-transition without a problem while no other mods were installed

i did crash once while swapping world, but that only happened while i was using all my other mods
so will take me awhile to work out what was causing that if i can actually replicate it