Here's the error that occurs on party join in WoW Classic:

Date: 2019-09-16 07:53:37
ID: 1
Error occured in: Global
Count: 1
Message: ..\AddOns\AutoGear\AutoGear.lua line 273:
Attempt to register unknown event "PARTY_MEMBERS_CHANGED"
Debug:
[C]: ?
[C]: RegisterEvent()
AutoGear\AutoGear.lua:273:
AutoGear\AutoGear.lua:196
Locals:

AddOns:
...lots of addons...
BlizRuntimeLib_enUS v1.13.2.11302
(ck=8e2)

Are you sure you're running the latest version? I can't reproduce this.

Running version 2.18.
Additional information: I am multiboxing 5 toons at once, if that would make a difference.

Synthetikaryote reproduced this. I think I just pushed a fix. Feel free to close this if it's fixed now. Thanks!

I'm pretty sure now this is fixed. Thanks. Please feel free to comment if it's not.