I cannot reproduce this problem reliably, but I've seen it happen several times already. FogBugz sometimes doesn't return correct answer on listWorkingSchedule XML api call. It simply returns empty response (<response></response>). Why does this happen, and how can I make it work properly? I'll try to update my question when I know more.
We just observed this problem on crime.fogbugz.com server (FogBugz Version 7.0.31H (DB 713, GEN 439, Build 968))
Update: after playing with Site working schedule in FogBugz UI, listWorkingSchedule started returning correct values. I am not sure which values were changed, but there were some good values already even before changing them.