Many server owners(including me) are currently experiencing issues with client disconnects. The client disconnects happen sporadically every 5-60 minutes and disconnects everyone in the area. From talking to a few other server owners I have deduced that the issue must be with a bug that was introduced in 7.0.0, and I also think that the fieldSize and Bad packet ID is being caused by conflicts with Astral Sorcery and the NullPointerException and IndexOutOfboundsException is caused by some other mod potentially. Below I've listed the admins I've talked to and what they had to say about the issue.

Admin 1(me): Has Astral Sorcery installed, gets Netty exceptions relating to fieldSize, NullPointerException, IndexOutOfBoundsException, and Bad packet id
Mod list and stacktrace(this is me trying to connect and disconnect multiple times, each time it disconnected me after 0-60 seconds and I get a different error): https://pastebin.com/WD3dRxz9

Admin 2: Has Astral Sorcery installed, he narrowed the problem down to a conflict between Astral Sorcery and Pixelmon and reproduced the issue on a fresh forge server with just those two mods. The version of Astral Sorcery does not matter. Gets Netty exceptions relating to fieldsize and Bad packet id. However, when Pixelmon is reverted to 6.3.4 the issues are gone completely.

Admin 3: Doesn't have Astral Sorcery installed, says that when he entered a different biome it disconnected him and everyone in the area, gets Netty exceptions related to NullPointerException and IndexOutOfBoundsException
Mod list and stacktrace: https://pastebin.com/yeLYtHvA

Admin 4: Has Astral Sorcery installed, did not narrow down the mods but was starting to narrow it down to Astral Sorcery
Admin 5: Had Astral Sorcery installed, after removing Astral Sorcery he had no more problems

Thanks for the report. I've passed this along to internal. We'll get this ticket updated when we know more.

From what I've been told, this should now be fixed for the upcoming version. It'll probably require some testing to be sure, but... let's see how this goes!

Just to clarify, This is a bug with another mod not using DataSerializer's correctly. But we dont use the DataSerializer mentioned in those stack traces anymore, so we just unregistered it. It is likely that bug will come up for you again but just not with pixelmon being in the stack trace.