Are you using the Anyconnect ISE Posture or ASA Posture (hostscan)? And what version?
If hostscan, what is the result you receive through the "debug dap trace" output? Windows Defender is a supported AV in almost all the hostscan versions. If this was working before and not working now, I would start by collecting a DART bundle and looking at the Cscan.log file under the Anyconnect ASA posture module section after a failed connection. Make sure to set the logging level for Hostscan to "Debugging" to get maximum information. You should some info like this:
[Tue Oct 25 10:26:35.837 2016][cscan]Function: get_data Thread Id: 0x1110 File: .\result.c Line: 505 Level: debug :endpoint.av["MicrosoftAV"]={}
[Tue Oct 25 10:26:35.837 2016][cscan]Function: get_data Thread Id: 0x1110 File: .\result.c Line: 505 Level: debug :endpoint.av["MicrosoftAV"].exists="true"
[Tue Oct 25 10:26:35.837 2016][cscan]Function: get_data Thread Id: 0x1110 File: .\result.c Line: 505 Level: debug :endpoint.av["MicrosoftAV"].description="Windows Defender"
[Tue Oct 25 10:26:35.837 2016][cscan]Function: get_data Thread Id: 0x1110 File: .\result.c Line: 505 Level: debug :endpoint.av["MicrosoftAV"].version="4.8.10240.16384"
[Tue Oct 25 10:26:35.837 2016][cscan]Function: get_data Thread Id: 0x1110 File: .\result.c Line: 505 Level: debug :endpoint.av["MicrosoftAV"].activescan="failed"
[Tue Oct 25 10:26:35.837 2016][cscan]Function: get_data Thread Id: 0x1110 File: .\result.c Line: 505 Level: debug :endpoint.av["MicrosoftAV"].lastupdate="11519511"
[Tue Oct 25 10:26:35.837 2016][cscan]Function: get_data Thread Id: 0x1110 File: .\result.c Line: 505 Level: debug :endpoint.av["MicrosoftAV"].timestamp="1465886084"
For ASA ISE posture, make sure to have the latest compliance module for the Anyconnect. Same procedure for looking for hostscan information using DART can be followed but the output maybe different.
Were there any Windows updates of late after which you are seeing this issue? I would also open a TAC case if all these steps do not give much information.
