zopress Shortcode publications highlight names with umlauts

Hello,

I would need help using zotpress Plugin in WordPress Blog to show publications with highlighted author names. According to shortcode help the syntax should be:

[zotpress collection="GMGCJU34" highlight="Sagan, C."]

Unfortunately, as soon as the author has german umlauts in the name, it does not work, e.g.: [zotpress collection="GMGCJU34" highlight="P?hlitz, J."]

Can anybody help, or give me advice where to look?

Thanks in advance,

Mike