Hi Erdem,
I can only suggest to try it once again after a complete reboot of the local computer of the user. I would say that the combination of the appropriate date format in both the SAP user settings (t/code SU3) and Windows regional settings should logically work. If not, then I suggest to create an SAP Support message for it.
Please keep us updated because I am curious about the resolution of this issue.
Thanks,
Sander