Ifcap is bad it is possible it has drain the battery and the owner never has checked the battery so ISS runs of the solar panel by day and dies at night.
Yes, that has to be what happens, at least in overall terms. Though without knowing (and understanding) the circuit details it's difficult to know the precise course of events.
But if you take the battery life in a 'good' (ie no damage to supercap or other related components) VP2 SIM board as 6-9 months with no solar panel attached (eg as in a 6372 Tx) then for a new battery to last only 1-2 weeks in system with a faulty supercap an unintended discharge path for the battery must logically have been created.