Many players are getting frozen in battle after a Pokemon dies with "waiting" message. This bug popped up in 5.1.1 (I never used 5.1.0, so unsure if it was there.) After many battles testing, there does not seem to be a specific catalyst for the glitch. However, it always happens after a Pokemon faints. This happened over 80% of battles we tested.

"Pokemon fainted" .... "waiting" ....forever.

This is happening in the main world and is not fixed when removing sponge GriefPrevention. Also has been noticed on other servers besides mine. When testing, two errors popped up, but they did not occur when the waiting glitch happened, so I am unsure if they are related to a different thing or not.
Hopefully this can be somewhat helpful.

https://pastebin.com/raw/NDJK9Bjg

https://pastebin.com/raw/GkFKChbP

we had waiting glitch too.. but we changed mods and then the glitch no longer occurred.

.. nm while i was typing it happened to my gf.. and we have almost vanilla.. ie 5 mods and thats including optifine and journeymap and craft guide. it happened after her charizard learned flamethrower + leveled up in a double battle vs trainer

we replayed same battle with single not double pokemon and no crash after lvling + flamethrower gaining

I have been able to replicate this with 100% accuracy. If a pokemon dies for a second time in a gym/trainer battle it will cause the game to be stuck waiting forever. In other words, if you revive a pokemon, put it back into battle and it dies one more time, you will have to end the battle using /endbattle.