the sum of setup, move and wait time exceeds the available time in the period

NAV 2013 R2

I have produced a released Production Order based on Sales Order Demand using Order Planning.

I go into the Production order routing and manually alter a start datetime having set all the ops to Schedule Manually.

I get this error: “The sum of setup, move and wait time exceeds the available time in the period”
Even if I change a date to its original value for example, I edit 20/02/2014 deleting the 20 and typing the 20 back in, I still get this error.

It is in the LoadCapForward function of the 99000774 Calculate Routing Line Code unit.

My aim is to update start and end times based on feedback from Preactor (a 3rd party capacity planning app). So far, I can do everything I need to do in my code unit, setting the ScheduleManually flags, altering the Work Centre to become a specific Machine Centre chosen by Preactor.

Now I want to change the Start end datetimes using calls to my codeunit. Trying to change the times manually gives this error, so I would expect the C/AL would get the same?

Is there a way around this? For instance, is there any harm in me commenting out this check in the Routing Line Code unit?

Is it something to do with my Work/Machine Centre calendars?

Thanks

Dave K