Patrick,

This is the first I recall hearing about an AT issue like that, but some have had the AP disengage issue you mention and there is a thread specific to it here on CS forums.

Important (from my experience) to use the AP Disengage bar on the MCP, and not a soft button or the Z key, as the default FS "AP Disconnect" seems only to remove the CMD button, which is not something you can actually do in a 757, the AP Disengage bar must be pulled out.

Others have stated it's related to the 4.40 version of FSUIPC and have reported that issue goes away when reverting back to 4.30 version of FSUIPC.

Are you having the problem just with the AP, or specifically with the AP LNAV/VNAV like Alcides was originally reporting in this thread?

If so, I'd certainly check your LEGS page in the FMC and make sure there are no discontinuities, and double check against PLAN mode on the EFIS display knob and "STEP" thru the LEGS page to make sure that the route looks sane on the EHSI display.