Do you have a backup file from before when this issue started? It could be from a long time ago provided your AMEX account is included in it. If so, restore that backup file and make note of what the Opening Balance transaction dollar amount is.
Then open your main data file, again, and look at the Opening Balance transaction dollar amount. If it is different from what you saw in the restored backup file change it to match what was in that restored backup file. (Also, enter the backup file dollar amount in the Memo field so if the transaction dollar amount is ever changed in the future you will already have the correct amount saved in the Memo field instead of needing to restore another backup file.)
Did this resolve the issue for you?
If not, here are a couple of other things to look for in the AMEX account register:
Is the issue now resolved?