There are here two descriptions of Frantisek's living: WHEN and WHERE. The accepted translation puts THEN last, but putting it after LIVED is perfectly correct and natural. Is there anything in the Czech sentence that suggests otherwise?
What's wrong with "František lived in a quite strange house back then"? The suggested solution is "František lived in a pretty strange house then", and I would assume that "back then" for "then" and "quite" for "pretty" both make sense, too