Managed to fix the issue - the original enemy group file name was "Hellish Crusaders" - apparently that was an inappropriate save file name *eye roll*
Anyways, fixed the enemy group, got it back the arc back up and running (Arc Id: 498667)
Yeah, this is an issue with an "overzealous filter" and has been a reported bug since April.
Good to hear you got your arc up and running again 
Craft your inventions in AE!!
Play "Crafter's Cafe" - Arc #487283. A 1 mission, NON-COMBAT AE arc with workable invention tables!
I am more annoyed that I had to go and "recreate" whole chunks of the enemy group.
Keep in mind that if you wipe your harddrive or get a new computer you have to copy the enemy group and custom critter folders over as well as the arc files. Otherwise, when you edit or unpublish you will lose critters.
Eva Destruction AR/Fire/Munitions Blaster
Darkfire Avenger DM/SD/Body Scrapper
Arc ID#161629 Freaks, Geeks, and Men in Black
Arc ID#431270 Until the End of the World
So I've had this AE arc up for over a year, and time and time again I've had to pull it down to fix whatever is suddenly "missing" while it is on live. Now the custom enemy group I have made is "no longer valid." I see that all the EB's and AV's have been wiped from the group and 75% of the minions are missing. Now that I've corrected those gaps AE still tells me "Custom Villain Group is Valid."
I sort of give up. I also see that one of my other arcs - the ENTIRE custom enemy group and arc has been wiped and is missing (after unpublishing it.)
WTF.