Describe the bug, provide any errors
When joining a 1.15.2 server where 1.9 bossbar packet is sent, 1.8 clients get disconnected with following message http://prntscr.com/vz66j2

How can we reproduce it?
Steps to reproduce the behavior:

1. Start a 1.15.2 server
2. Install latest build of all via plugins and a plugin that sends bossbar packets
3. Join with 1.8 client

Expected behaviour
Bossbar packets translated into entities

Screenshots
http://prntscr.com/vz66j2

ViaVersion Dump
https://dump.viaversion.com/ada4cd4e9b665005ee5736f64362fea7472ac4b8e8cfc3a48630c7f4f38e28b2

Additional server info

Checklist

• I have included a Viaversion dump
• If applicable, I have included a paste (not a screenshot) of the error
• I have tried a build from https://ci.viaversion.com/ and the issue still persists

I can't replicate this on a 1.15.2 server nor a 1.12.2 server. Perhaps I need a config change or something in TAB? I enabled bossbars in the default bossbar.yml

1.15.2 - https://dump.viaversion.com/f877f5403fc7fd6b3282d6bafbd798fd5bf3ba1c4c8fbc7d377aaa6cd7f4c01b
1.12.2 - https://dump.viaversion.com/5e9024233637a5776a83d1708be992fc6090009031f38f8faba9cf04070f43ba

Turns out to be caused by online-mode=false in server.properties file. With true it works. No idea how or why, but this indeed is the cause of the disconnect when a bossbar packet is sent to 1.8 on a 1.9+ server.

I was able to successfully reproduce it with a plugin as trivial as this https://pastebin.com/2wMxjwij.
Via dump https://dump.viaversion.com/4f67b7659a1a244476c401abfc15bf214292416a85498d1d3fa2b7d72f1a107b (the "test" plugin is literally the paste above).
Apparently this affects more server versions. Just tested 1.12.2, 1.13.2, 1.14.4 and 1.16.4, still disconnected.
Disabling the plugin sending bossbar works http://prntscr.com/x80vwk.

I tried with the plugin provided and it doesn't seem to error still, I downloaded older Via* jars to match yours and just seems to work fine.

Here's my dump - https://dump.viaversion.com/f37a7060814f7af48d0f1d857ebaa1f28946349f2e466d273ea1080675f2dcea (also tested Java 8 in the case it was that)
Image - https://i.imgur.com/9YGaFzL.png

Looks like this is a duplicate of #222 and possibly #243 .

Thanks for the info, based on my current findings:

Works on ViaVersion 3.1.0, ViaBackwards 3.1.0, ViaRewind 1.5.0
Issues on 3.1.1, 3.1.1, 1.5.1 (unsure if just bad mappings, might be my end)
Error appears on 3.2.0, 3.2.0, 1.5.3

Commenting out updateLocation in WitherBossBar.java seems to cause the issue not to happen. Not really sure what the cause is, unfortunately (may be related to a change in ViaVersion/Backwards).