How is [x] used in medieval German?

[Koen G]

Also notable is balax which is same as balas (blass) and thus supports Aga's idea expressed in the other thread that -x as the word ending may be variant of -ss, that is e.g. "marix morix vix" may be read as "maris moris vis"

Yeah but his Latin didn't make much sense  I'd also wonder why they spell specifically ballax with x.

Several of the words you list are clearly Latin loanwords. The others are specific words that somehow ended up being spelled with "x". The word for six, "sex" is a common one indeed, as are certain others like ax and ox. 

Now of course the most interesting one is "pox" for "bocks" in Fastnachtspielen. If memory serves, those are also all expletives?