Sorry i did not include this. I have version 126.96.36.199. running on window 10 home.
I have just experienced this again, working at my computer. I started a task on WorkingHours, saved it and ran it in the background while working on a word document. At some time approximately one hour after starting, I checked the time tracking and it had stopped of its own accord. There were no other programs running in the background apart from Outlook.