Must I make up for the missed fasts (qada') or offer expiation (kaffara) if I intended to fast five consecutive days as a vow (nadhr), fasted one day, then broke my fast the following day due to uncertainty about the intention of consecutiveness?

Your fast on the day you doubt whether you formed the intention to fast it overnight is valid, and fasting without it would not suffice for you; you are obligated to make it up. For an obligatory fast, it is necessary to form the intention from the night before.

If you are not certain that you intended to fast the days of your vow consecutively, then it is permissible for you to fast them separately. However, if you did intend consecution and it was interrupted, you must restart.

You must be careful about excessive doubts and hesitation, so that they do not lead to obsessive thoughts.