I am now experiencing precisely the same problem but with a Supermicro X10SAT motherboard, hosting an Intel Xeon E3-1275 V3 CPU (no external GPU).
vPro/AMT KVM was working fine until today when I unfortunately updated the BIOS to R3.0 (5/26/15). Now the AMT KVM behaves just as gr8ape describes -- no video transferred (bytes received count at 76) and keystrokes reaching the server until the connection terminates with the internal KVM failure.
I've already logged a case with Supermicro and will report back any result. Worst case, I'm hoping Supermicro can at least share older BIOS revisions so that I can revert
Thanks,
Don
Edit: Kin Yan from Supermicro support replied to my query within 15 minutes and provided a new (beta/internal) BIOS image named X10SAT_T201509031611.ROM. This update seems to have resolved the issue completely, so if you're using another SM motherboard, you may want to contact their tech support. I'm quite impressed with their speedy and helpful response!