small AC commutator motor issue
When you permanently connected the field coils, you must have reversed one with respect to the other. In conjunction with the armature, one field coil is trying to make the motor turn in one direction and the other is trying to make it turn in the opposite direction. The result is a "frozen" rotor.

If the problem is the two field coils magnetically opposing each other, the correction would be to transpose the two wires of one field winding.

I think that a reversed field winding connection the most likely problem, but it is possible that there is some looseness in the mechanical construction that allows the rotor to move and mechanically bind up when energized.

Note that reversing the armature connection with respect to the field will reverse the direction of rotation.

Failure Due to Excess Speed

It has been suggested that a universal motor that is tested without a load might fail because of excess speed. The speed of a universal increases drastically when the load is reduced. With a vacuum cleaner, blocking the air flow stops the air flow and reduces the load to nearly zero because the blower is stirring the air internally rather than moving a mass of air. You can hear the sound of the motor increasing in volume and pitch when that happens. That accelerates the wear in the motor bearings and commutator. It also increases the stress on the rotor parts.

When the motor is removed from the driven machine, there is a danger that the speed will be excessive, but the speed is still limited by the commutator and bearing friction plus the aerodynamic drag of the rotor parts, often including small fan blades on the rotor provided for internal cooling.

If a motor fails due to excess speed, it is unlikely to die quickly or quietly without leaving evidence of failure. If anyone wants to see and hear what that kind of failure looks and sounds like, demonstrations can be found on you-tube. If you want to experience the smell, I recommend that you burn something that is unlikely to fly apart and hurt someone.