One thing not mentioned that can cause date issues like this is Regional settings. Probably they are the same from pc to pc if that those options are set via policy then locked down. But perhaps as an admin, he/she has altered the date format for their profile (or is that a machine level option? I forget).
As for the message, it is saying one field was set to Null but it seems the record was allowed to be written, so the table field must be set to allow Nulls. The missing date value might help you zero in. It could also be the way the date is being entered by the user, but I can't tell from your description of the logged in tests. You might need to check out that possibility as well as the regional settings before doing anything drastic like reinstalls.
The more we hear silence, the more we begin to think about our value in this universe.
Paraphrase of Professor Brian Cox.