Plastiq reached out to let us know that they started again allowing payments with an Amex card. For the past couple of weeks it hasn’t been working, but it’s now been restored.
As before, there are still more restrictions with Amex than others, for example, you can’t pay a mortgage with an Amex card. It seems they broadened the restricted payees a bit too. They updated the terms (4.1C) to indicate the following:
For Recipients in the United States, the use of American Express cards is permitted for the following industries: Government, Utilities, Education, Residential Rent and Club Fees and Memberships.
Check this Frequentmiler post for a handy chart of what works and what doesn’t now.