Short answer. Bosch error 500 means the drive unit detected an internal electrical fault. Turn the system off and back on. If it clears, ride cautiously and monitor. If it returns, the drive unit needs a dealer diagnostic and often a firmware reflash or motor replacement.
Step by step
Power cycle the system. Turn the drive unit off, wait 30 seconds, and power back on. About half of error 500 events clear this way.
Check the motor cable connection. Look at the cable between the motor and the frame for damage, water intrusion, or a loose connector.
Update the firmware. Newer Bosch firmware fixes several error 500 causes. Any dealer or mobile tech with the diagnostic tool can update.
Get a diagnostic if it repeats. Repeating error 500 usually means a motor internal fault. Continued riding can accelerate the damage.