Re-installing BUG didn't solve the problem. I used the singleplayer installation.How did you install BUG? Single- or Multiplayer? Did you try to re-install BUG?
What other mods do you have installed? How and where did you install them? Or did you do a custom merge?
BUG uses its own CvAppInterface module that calls fireEvent(), so that file seems to be working okay. However, BUG does not use CvCustomEventManager that some other mods use. I wrote BugEventManager that adds fireEvent() and other features. Somehow some other mod is injecting its own event manager into BUG.
The OOS is, like you guessed, the multiplayer fix mod, but I haven't installed it since I reinstalled the game, earlier today.The odd thing is that with BUG in CustomAssets, it could interfere when playing other mods (Next War for example), but if you are playing the normal game, other mods shouldn't interfere with BUG. Did this suddenly start happening after using BUG without problems for some time?
How are you installing BUG now? Are you using the EXE installer or copying your old installation? If the latter, try reinstalling BUG using the installer. It will have an option to rename the CustomAssets folder; use that.
What is OOS? Is this the OOS multiplayer fix mod? That shouldn't have any problems as I believe it is only a DLL with no Python files of its own.
I must have done something wrong: the CustomAssets-Renamed-BUG-3.6 folder is empty.
I tried loading Next War. This time a lot more Python Exceptions visited.
This is expected as I said. BUG installed into the CustomAssets folder will interfere with other mods unless you tell those mods to ignore the CustomAssets folder. Go to the Next War folder and open "Next War.ini" in Notepad. Look down a few lines for "NoCustomAssets". Change it from 0 to 1. You can do the same for all the other mods you play.