I haven't run into this... but it looks like military addresses have a "state" value of:
AE, AP, or AA
and then they have a city value of:
APO or FPO
It looks like this client entered a bad value for the City. They entered "AE", but that is not a valid entry.
I don't have a standard way to account for this. We could probably test if an email address shows up as valid in google and then add an error message if it doesn't. Entering a non-existent city name isn't unique to military customers, but that is essentially what the issue is.