Reloading the save always ended up with the same result. I could advance to turn 165 from 164, but upon attempting to go from 165 to 166 I get a crash during what appears to be Pahlava's turn.
However, after doing a search for all instances of [error] using Notepad++, I found these lines of code:
Line 153190: 17:40:17.370 [game.script] [error] Script execution error for <spawn_army>, at line 15626, in mods/ebii/data/world/maps/campaign/imperial_campaign/campaign_script.txt:
Line 153195: 17:40:17.370 [game.script] [error] Script execution error for <move>, at line 15628, in mods/ebii/data/world/maps/campaign/imperial_campaign/campaign_script.txt:
Line 163068: 17:42:20.527 [system.rpt] [error] Unspecified error. Please compress your error log in a zip format and upload it on the EBII technical help forum.
This led me to blieve the problem may have had something to do with Bithynia (settlement of Nikaia). I loaded to save, toggled FoW, and Nikaia/Bithynia is Eleutheroi but there is a Pergamon stack nearby within the territory. When advancing the turn from 164 to 165, during Pergamon's turn the stack besieges Nikaia. I do my thing as Bactria, and then as I got from 165-166 I get a crash again (but again, during what appears to be Pahlava's turn based on the centered faction symbol at the top of the screen when the crash occurs).
Looking up the campaign.script I see that the script execution errors have something to do with spawning an army when the territory of Bithynia/Nikaia is controlled by Eleutheroi. So at this point I thought that maybe if the territory wasn't Eleutheroi the crash wouldn't happen. To try to remedy this, I loaded up my save and transported one of my armies using console commands so that I could attack Nikaia myself on Turn 164 (the current turn when I load my save). To my surprise this resulted in a crash as well though as soon as I tried to advance. And, strangely, when I looked up the log to see what [error]s appeared this time around, there was nothing but the script errors in export_descr_buildings.txt at line 25218 (which appears all the time) and the unspecified error.
At this point I've reverted back to an earlier save. Toggling off FoW reveals that Bithynia/Nikaia are currently owned by Pontus so hopefully history takes a different course with this save and it doesn'[t go back to Eleutheroi (if it was indeed something to do with that that caused my crashes).