Problem with context-sensitiv help

I'm actually trying the single-source-workflow FM -> RH. In my RH-project I import framemaker content and create new topics based on the name of the headings. The headings contain Umlaute (ä,ö,ü,...) and the html-files which were created through the import process contain them too (e.g. Strecke_einfügen.htm). Unfortunately I can't call a context-sensitiv topic which contains Umlaute in the file name.

I think this is a bug. To test the behavior without single-source, I created a topic in Robohelp 7, named it "öööö" and tried to call the topic with the tool csh-test. No reaction. I tried the same in Robohelp 5 and it works fine.

How can I solve this problem?