On PMP320 the number of packets with errors on "Ethernet Packets: Tx" increments all the time. This can be because of bigger size of MTU configured on the end devices, i.e. router, switches etc, connected to PMP320 radios.
By lowering the MTU size on end device i.e. router, switch etc will fix this problem. Lower down the MTU size step by step and monitor the Packets with error on PMP320 AP, set the value of MTU where packets with error stop incrementing.