Sounds like it could be one of multiple possible issues: 1) bad panel, 2) bad motor, 3) bad wiring in between. The low open terminal voltage does not sound good. But how much current (Amps) is required by the motor to run? Is there anything on a name plate or information in literature? Is there an on line link to information about this system to look at?
You can think of the solar panel as a DC power supply. If it is dying or has an internal problem then it may effectively have built in resistance and the voltage drop during load (current flow) is internal to the panel. On the other hand, if the cables are corroded at the connections then this would also represent a resistance and the voltage drop could occur there. Where do you measure the voltage in these experiments. At the motor? These connections should be getting, at least a little, hot. Either way, the motor would not be seeing much current as there would be little voltage appearing across the motor terminals, but if there are bad cables and connections the voltage should still appear at the output of the panel. Or, perhaps, the motor has died and is shorted internally. If there is a high amperage flowing the motor should be getting hot to the touch. If you have two car batteries available you could connect them in series to get to 24 volts and they should spin the motor. However, if there is an internal motor short these batteries may supply enough current cause a fire. So only connect them long enough to see if the motor runs. Since, you got the motor to run a little, it sounds like it is just not getting enough current.
