Export Reports at Greek language gives wrong characters

Hello guys,

I am on v7.12.4 and when i am trying to export some report (or anything actually) the exported file is opening with wrong encoding. I am using UTF-8 (tried also the ISO-8859-7) which is supported for the Greek language characters but the results are in weird symbols when i am opening with Excel. With OpenOffice it works fine tho (and with the import data via csv option in the Excel, but not with just opening it, as it should i guess).

Any idea how to fix that?
Thanks in advance for the help!