Success in reauthorizing and connecting out 2 Amex accounts (3 cards). Slightly different from the post from @DJPhilouis on Mar 1:
This is in Win 11 23H2 Build 22631.3296 with Quicken R55.15
Unfortunately, I cannot guarantee this will fix this issue for everybody. If it helps a few troubled Quicken/American Express users, then it will have been worthwhile typing all of these steps out.
Good Luck!