The curious case of an an empty date

I have something strange going on.
I made a report to show a list of items with no ending date entered.
I used the double single quote ('') as a filter value and got what I wanted.
Nothing spectacular there :)

Then I shared the report with someone else and she isn't able to get the same results.
There are 25 items in the table, ALL have no ending date (the ending date field isn't in any key, btw)
When I run the report I get all the records, her report ends up empty.

Without changing the report I added 5 ending dates (all 31-12-2016) to the database en refreshed the reports on both setups (which are the same version of excel and jet btw)
I get 20 records, she gets nothing.

I altered the report to filter on not empty (<>''), I shared the report and we both get the same result: 5 records.
Changed it again to filter on empty or after a certain date (''|>1-1-2016), I get ALL, she gets 5.

Does anyone know why filtering on an empty date with the same datasource, the same jet version, the same excel version results in something different on 2 machines?
What weird regional settings or windows localization may cause this? There has to be something different somewhere….

rmw