kolab syncroton and german umlauts (umlaute)

[User, Kolab]

Hi Folks,

if I receive E-Mails through kolab-syncroton (no matter on which 
device/client, eg. iPhone; Microsoft Outlook,..) the german umlauts are 
not displayed (eg "Weihnachtsgrüße" shows up as "Weihnachtsgre". If the 
E-Mail is viewes within Roundcube everything workes fine.

Any ideas on how to fix this?

I have the CentOS Packed Version of Kolab/Syncroton in use.

Thank you for your response.