I use a portable installation of ScreenshotCaptor in Dropbox so the same settings are used across all of my computers. At work I have two large monitors and at home I have a smaller one. Last night I could not see the main window of Screenshot Captor and I couldn't figure out why. Screenshots appeared in the screenshots dir as usual, but no amount of cajoling would make the window appear. This morning I ran it at work and the main window was on the second screen. I then realized the saved position was off screen at home, and confirmed it by manually changing UserMainLeft and UserMainTop.
Could you make sure UserMainLeft and UserMainTop are within the current screen resolution?
As a stopgap solution I added a new entry to my AutoHotKey Window movement shortcuts:
; Move the active window to 0,0 in case it's off the screen with Alt+Windows+Home