I loaded up my server on 1.15-pre1 after backing up our 1.14.4 world.
I was testing the various dimensions and in the end everything seemed to be fine except as you go towards the end gateway (within 10-20 blocks) your game crashes. My friend and I tested this each three or four times and it was consistent that the game completely crashes when you walk near the gateway.
Duplicate of MC-166004