Report - Group field error

Hello,
I'm still working with ASPR 6.1.

I generate a report and get a list of entries grouped by event id / organizer / entries per week.

Unfortunately ASPR is not allocating correctly the grouptitle (i.e. week from Day 1 to 7) to the according dates of the entries.

Here you can see an example:

Example
Go to the group title 04.01.2010 - 10.01.2010

Here you can see that entries for 14.1. and 15.1. are included.

FYI: in Europe we have notification of DD.MM.YYYY instead of DD/MM/YYYY.
The strange thing is that the same project works when I set Language / Regional settings to english - with all other languages I tested the error occurs.
Is this a bug or do I have to change something in the settings?

BTW: I tried the demo version of 6.2 with same result.
Regards,

Philip