Yes, this is a common problem which has been reported before. It is caused by one (or more) of the units enraged by the Avatar, not by the Avatar itself.
Thanks for your reply. Yes, you are right, I opened the WorldBuilder and found lots of units with the enraged promotion. But since I am playing on an extra large map, there are probably hundreds of such enraged units and it would take too long to remove the promotion from each unit one by one, unless there a possibility to do a 'change all'.
I am really surprised that such a serious bug has managed to survive in this superb mod. Is there a known workaround like tuning off the 'Enraged' event when the Avatar enters the game?
UPDATE:
I was able to find a workaround to prevent units from becoming enraged when the Avatar of Wrath appears.
In the file CvRandomEventInterface.py located in the entrypoints folder, change the following line in the doArmageddonWrath function definition paragraph.
Change the line from:
pUnit.setHasPromotion(iEnraged, True)
To:
pUnit.setHasPromotion(iEnraged, False)
Yes, this is primitive and other people have probably figured out this fix as well but by applying this change the turn before the Avatar of Wrath showed up, none of the units became enraged and I was able to keep on playing without any endless AI turn problems. Anyway, that's my 2 cents.