Ah, but you were on the right track anyway.
Even though WordPress complained (and highlighted in red) the name of the category when I tried what you suggested, I went on and changed the slug for my C++ category from “c” to “cplusplus”.
After that I could do what you suggested and now my problem is solved.
But it seems to me the slug is used to check if a category name exists. Which can be somewhat misleading (at least it was in my case).
Thanks for pointing me in the right direction ??
edit: Also, a similar problem seems to exist when it comes to tags. It is impossible for me to tag something with “c#” it will result in being tagged with “c++” ??