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If I understand correctly the breaker is tripping as soon as the final battery connection is made, and not when the scooter is being driven. Assuming that the polarity is correct between the battery pack and controller then if the circuit breaker is blowing when the final connection of the battery pack is made that indicates a possible short circuit in the wiring between the breaker and controller, or internal problem with the controller. When circuit breakers wear out their trip rating does not change by that much, so installing a breaker with higher Amp rating probably would not help. A 48 Volt scooter with 500 Watt motor usually uses a 30 or 32 Amp breaker, when overvolting a 48V 500W motor to 60V its new power output becomes 830 Watts so a 50 Amp breaker would be needed. If changing from a 30 to 50 Amp breaker the wiring between the batteries and controller should be a minimum of 12 gauge so if it has thinner wire then that it should be upgraded at that time.

I do not recommend using a self-resetting circuit breaker like the one in the attached photo because it will reset without giving any warning which can be dangerous on an electric scooter. You really want to be in control of when the circuit breaker resets so you can make sure everything is okay after it is reset.

If the breaker is immediately tripping when the fifth battery is connected then the fifth battery may possibly not be wired in the correct polarity with the other four batteries. I would check to see which wire under the seat goes to the positive output lead from the 48 Volt battery pack and make sure that this wire goes to the negative post on the fifth battery.