Calculated field no longer works and gives a value error

Good morning,

So here's today's issue - I have a SharePoint list that uses a calculated value to determine the total training time using the formula =TEXT([Heure de début de formation / Training Start Time]-[Heure fin de formation / Training End Time],"h:mm:ss"). The data type for this formula is Number and both training start time and training end time are Date and Time fields.

This is supposed to return a value such as 1:30:00 and it did for quite a while. But now, the list entries display total time as 1 (instead of the previous 1:00:00), 0 (if the training was less than an hour), #Name?, or #Value!. Anyone have any ideas as to what happened and how to fix it? 

Thank you