This looks like a language problem. I tried to reproduce but it was good (in English, French and German versions). See pic for English version. Which Joomla! site language are you using?
As explained in the article http://www.jdownloads.net/documentations/item/international-dates-times-and-number-style
jDownloads uses the main Joomla language file for the THOUSANDS_SEPARATOR definition
I tried with the English French and German site Languages and it was fine (although the French one wrapped on the spaces)
You could also try changing the Short and Long date- time fields (I forget which one is used!)
Does this happen on the Download Details view?
Have you tried the other Modules to see if problem is wider?
Thought: in public static function strToNumber it could test if returned string is > 1 char and use default if it were a long string. That would then work for partial language translations.