C20 indicates that the drum cannot move in the Hitachi service chart consulted. The load and suds are usage conditions to check, but the alert does not prove a broken motor.
The official Hitachi help links C20 to drum movement and suggests considering too many clothes or suds.
Check the conditions without forcing the spin
Stop the cycle and wait for a safe opening. Check whether the load exceeded the program capacity or whether there were unsuitable bulky items. Do not force the drum to turn to overcome resistance, and do not insert tools into its openings.
If there was a lot of suds, note the detergent and dosage. Do not add a homemade product to get rid of it, and do not keep drying or spinning without following the recovery steps in the manual.
If C20 returns after the usage conditions have been corrected, request a drive diagnosis. Note whether the drum never started moving or stopped after beginning: that detail is useful, but it does not separately identify the motor, transmission, or control.
Stop chaining restarts when the movement is still blocked; report C20 and the load that was present when it occurred.