Not sure if goliath is running into the same problem as I am, but I have not been able to use AMEX at Starbucks for the past 2 weeks. Tried at Curve (2 outlets), Tried at 1U (2 outlets).

Partly, yes. 

Other than that, I've tried using it over the weekend at Pavilion (6th floor) and Mid Valley (ground floor and 2nd floor at The Gardens). And at both locations, they were having issues with Amex payment. 

So, I am wondering if they have stopped accepting Amex or it's just both of these locations having issues with their terminals.

Yup it's correct. After keying all the details, it won't take me to a page where I need to enter the MSOS code, it straight gives a message "transaction is cancelled". Is there some sort of "activation" needed for online transactions?

Have you tried watson online?
just now i tried watson online
it use ghl gateway (e-ghl.com)
but 3 attempts (failed) redirected me back to watson payment page after key in my card
called maybank cs no record of attempts
changed to use quantum ghl also use maybank gateway to call pb gateway
(so i assume e-ghl use maybank merchant)
then successful
strange