Use any rigid, flexible, monocrystal, polycrystal, etc panels with 6V rating (12 'cells') for the best performance
We've tried 5V panels and they don't work. 5.5V panels sometimes work, 6V is really the best!
This is a fault condition detected by the charger. Most often it is a battery problem. This can happen if the battery is allowed to discharge completely. The protection circuits on some LiPo cells require the battery to be disconnected in order to reset the circuit after an over-discharge cutoff event. Often just disconnecting and reconnecting the battery will allow it to charge again.