1.16.5; My friend and I were playing as a sea dragon and cave dragon respectively. When we got our wings and activated them to fly, we both crashed within a few moments of taking off. Flight in Creative mode didn't cause any crashes.

crash-2021-05-09_14.59.06-client.txt

@Jorgee9001 confirm whether it was caused by Fins and Tails mod, otherwise I will close this issue in 3 days.

Just double checked. Seems to work fine when Fins and Tails is removed. Edit: Forgot to mention that I brought this to the attention of the F&T Dev too. So hopefully a fix will be in the works.

From what I can tell, this crash is somehow related to the fins and tails mod. In their playerTick method they handle some sort of custom flying packet (which is null). I don't know the reason for this but from what I can tell it is the cause of the crash.