Special characters (e.g. umlauts) completely missing when exporting after XWiki update

After an update of XWiki from version 10.7 to 10.8, special characters are completely missing when I export any wiki page to any of the formats (PDF, ODT or RTF). HTML however works. Missing means that there is no placeholder or anything, they're gone like if you delete a character.This is very annyoing in general but also due to the fact that we are located in Germany. 

Looks like an issue with character encoding. It might be some old configuration file that didn't get updated, or a general issue with 10.8.

Where should I start looking?

One example: The text

äöüߧ$%&/()=?`

becomes

$%&/()=?`

when exported to PDF.

Cheers