I have the exact same issue as described in those two tickets : here and here.
We implemented transactional messaging on our Staging ACS instance, where everything works fine. After moving to production, we get the following error message when calling the transaction message API. (The token exchange works fine) :
"message": "Oops. Something went wrong. Check your URI and try again."
After seeing the response in the two tickets I referred, I created a second projet in I/O, specifically for Production, but I still get the same error. This doc seems to indicate it might be a CNAME issue, to check with support. Can this really be the issue? Does someone have an idea of what else might cause this?
SOLUTION EDIT : A separate Adobe I/O project is indeed mandatory for staging and production. The 502 issue in production was indeed related the a misconfiguration with the CNAME, which support could fix for us.