I had the same issue.  Did all the "fix" suggestions myself before coming here and none of them worked. Called customer service from the chat.  Logged a problem after being on the phone for 1 hr+.  Never heard back on my case.  5 days later amex "worked" for 2 days and for the past 7 back to "Try again later".  This is NOT an AMEX issue as has been pointed out by several people - we can log into AMEX no issue.  Something is wrong with Quickbooks and nobody is addressing the issue clearly.