I have a HBLink server thats been running for many months now on a cloud server, about 2 weeks ago it just stopped connecting to brandmeister.
On starting it manualy I noticed an error coming back MSTCL Recieved the same as in your posting.
I have repeater freqs set in the connectors to BM and also my own Password login set as it allways has been and was running.
The only way I could get my system back going was to image the server, start a new server with a different IP address and put the image back on the new server start it and it all worked instantly.
From this I got that MSTCL Recieved means the IP address has been blocked.