Yes, very likely the issue. SGP needs to know when the camera is in "Mirror Lock" mode as we have to fire the shutter an additional time. So if the camera was in mirror lock but SGP didn't know about it (by having the Mirror Settle Time be non-zero) then we would fire the shutter only twice. Once to start and once to end. However with mirror lock on the camera that gets translated to "Mirror Up" and "Bulb Start" so we never end the exposure...so there is nothing to download.
Thanks,
Jared