Product Specification Attribute with special character or umlaut

We created several product attribute specifications and corresponding values and allowed for filtering.
A problem arises if the specification name contains special characters or german umlaut (e.g. 'Ø mm', 'Ausführung').
As soon as we try to filter by that specification, a browser error page is displayed stating 'The page cannot be displayed'.

The URL at that point looks something like this : ../category/91-gerate.aspx?ausführung=7227

Looking at the way the url is modified/constructed in other places of nopCommerce I adjusted the url manually to (simply replacing the 'ü' by 'u') :

http://new.pricon.de/category/91-gerate.aspx?ausfuhrung=7227

After that the page can be displayed and the filter is applied correctly.

NopCommerce 1.80
SQL Server 2008

It would help if anyone could point me to the right direction to explore how url rewriting works.

I think you can't have special characters in any kind of url.

You probably can using special notations. But the point here is that nopCommerce builds the links for the filters WITH the special characters. So if you hover over the filter you'll see something like this:

.../category/104-mikrofaser-brillenputztucher.aspx?größe=7716

But that url does not 'exist'. What actually CAN be mapped (discovered by playing with the url) is

.../category/104-mikrofaser-brillenputztucher.aspx?groe(mm)=7716

It seems that while the special characters are removed in the url rewrite the actual target urls of the filter control still contain those characters.