That is probably the result of allowing word combinations in Dutch and bak and back are both allowed as parts of a compound word (bak-cup-systeem) for systeem because the words as such aren't in the dictionary.

Actually the correct would probably be "back-upsysteem" comparing with "back-upbestanden" present in the Dutch dictionary although that looks weird (back-up shows at a lot of places).

The correct word is indeed "back-upsysteem". But is there a way to remove the words "back" (is not a dutch word) and "cup" (I rarely use that word) from the dictionary?

Those two words show as cup/CcZaCACBYbC0 and back/CcCAZaCBC0 in the file nl.dic in the dictionaries folder in the Firefox program folder or as an installed extension in the Firefox profile folder if you had to install that dictionary yourself.
The file uses Unix line endings (LF instead of CR/LF), so you need to use an editor that can preserve those line endings.

What editor can I use for editing that file? Although I don't think I will edit that file because I see a lot of english words in that dutch(!) dictionary... I mean what is a word like "cupboard" doing in that file... And what will happen with the dictionary when I update Firefox? Thanks for your help. I think I just have to stick with incorrect spelled words and I just have to add the correct word(s).