You're right. When i do an export of 1 user, delete this user and import the generated csv file it works fine. I'll figure it out and report here.
Thanx.

The "save a copy" option from LibreOffice does the trick. Settings are:
- character set: Unicode (UTF-8)
- Field delimiter: ,
- String delimiter: "
Save cell content as shown: checked
Quote all text cells: checked
Other options: unchecked

Thanx for the hint Kyle.