According to https://en.wikipedia.org/wiki/Wikipedia:Naming_conventions_(technical_restrictions) for page names, "ø" is a valid char in pagenames, and I'm asumming this is also valid for category names since nowhere in the documentation it's stated otherwise, however this char breaks category names in mediawiki 1.26.2: any page bound to a category with "ø" as part of its name will never be listed as part of it. Try for yourself, or see here for a real life example:
As you can see, the category appears to be empty, however if you enter "test1" or "test2" in the search box, you will land in two pages that ARE part of the category.
There may be more problematic characters that aren't mentioned anywhere but I found a problem with this one in particular as I'm using a related category already:
which at some point worked at it should, yet right now I can't make new pages appear on it.