Maybe try adding textboxes for each total one at a time e.g.

me![tot6] =DateDiff("n",Nz([START TIME6],0),Nz([Finish Time6],0))/60
me![tot7] =DateDiff("n",Nz([START TIME7],0),Nz([Finish Time7],0))/60
etc

that might show whichever field is causing the problem.

If that helps you fix it you can then use, e.g. =[tot6]+[tot7]+[tot8]+[tot9]+[tot10]
to get the overall total (which is also a lot easier to read.)