This action is no longer available since this connector was removed.
httpPOST action in message card not working
Indeed, it looks like the problem has been resolved. Our card actions are working as expected again.
It started working again for me for cards created from around August 4th onwards.. I have cards created on 31st July that still show the same issue though when submitting the action.
Brady Vidovic commented
Same exact issue here, started at the same time (July 18, 2018)
Office 365 support confirmed the outage, that there is no known workaround, and that this wasteland of a bug tracker is the correct place to report the problem.
We started seeing this today with an Incoming Webhook connector that's been working for months. The connector has definitely not been removed.