Looks like it's a conflict with trying to write to the same filename simultaneously by multiple process at the same time. Let me reproduce it and then add a few different ways to fix it.
But try this in the meantime: Add $uniquenum$ to your filename, such as:
minicap.exe -capturedesktop -cursor -compress 5 -exit -save "C:\MINICAP\%COMPUTERNAME%\%USERNAME%\ $dt:%Y %B %d %H-%M-%S$ $uniquenum$.jpg"
I'm not sure that will solve it, because the problem may be that the file is being created by another process between the time it checks for an existing file and the time it tries to save it, but give it a try and let me know.