ActiveSync is a wonderful thing — unless something goes wrong. In the maddening manner of most Microsoft error messages, you get the functional equivalent of “something went wrong” with no other detail — and worse, an error message that appears to be telling you something:
“Critical communications services have failed to start. Try resetting the mobile device, and then connect again.”
This might lead you to conclude that the issue is on your mobile device, when in fact, it appears to mean nothing of the sort: it appears to actually mean, “ActiveSync didn’t receive any communications from the device” which is just as likely, if not more likely, to be a problem on the PC side.
ActiveSync communicates on these ports:
TCP from Mobile Device to PC: 990, 999, 5678, 5721, 26675
UDP from PC to Mobile Device: 5679
So you can start troubleshooting by making sure these are open and available on the PC. (If you have a “personal firewall” on the PC, start there.)
In my case, I finally traced it to Winsock2 corruption, as explained here. Something I’d installed or deinstalled apparently managed to leave a wake of destruction.
The solution was to open a command window and execute “netsh winsock reset”, then reboot.