This is a traditional, no limit, AmEx card, not one of the more common consumer credit cards they've started offering in recent years. He's been traveling for multiple weeks of out the past 6 weeks and doesn't return to the US for another week and a half, so first, there's a shitload of expenses on that card, and second, he still needs to put another shitload of expenses on that card before he returns home.
So, what happened? According to AmEx, while their traditional cards have no limit on travel expenses, their traditional cards have and have always had limits on retail purchases. Okay, we're not even going to question that concept. But he's been traveling on this card for years, often gathering retail expenses in excess of what is currently on this card.
First, they tried to claim that his retail limit had always been what it is, and that he'd just encountered it for the first time. Only, the conference fee which is now "blocking" his card, he's put on that card for the past 5 years without a problem. And the conference fee ITSELF is over the limit they're claiming his card has always had.
Then, they tried to claim that his company explicitly requested that his limit be lowered to the limit that now exists. His company is currently WTFing at AmEx, and probably running around like chickens with their heads cut off, because my husband is nowhere near their most extensive business traveler, and if AmEx imposed/lowered this limit on all their employees across the board, all hell is probably about to break loose. Over Memorial Day Weekend.
AmEx is also refusing to raise that limit in order to carry my husband through the rest of his business trip, and is trying to insist that his company pay off expenses on the card immediately if they want his card to function again for non-travel expenses.
Also, please see the first sentence of my post. Apparently, purchasing a ticket on the London Tube is categorized as a "retail" purchase, not a "travel" purchase.