Had the exact same combo on a MR-J4 driving a 150T press last year. AL.16 plus AL.20 together usually means the encoder itself is dying, not just the cable. Here is why: AL.16 is communication error (could be cable), but AL.20 is encoder internal data error. When both show up, the encoder is sending corrupted position data intermittently.
Before you replace the encoder, check one thing that bit us: the connector on the motor side. Pull it off and look at the pins under magnification. On high-vibration presses, the solder joints on the encoder connector crack over time. We had pins that looked fine visually but had hairline cracks. A reflow with a soldering iron fixed it for $0 instead of a $2,400 encoder replacement.
If the connector checks out, then yes, replace the encoder. But order the exact Mitsubishi part number for your motor — aftermarket encoders for MR-J4 exist but the timing calibration is different and you will chase position errors for weeks. Ask me how I know. Also, after encoder replacement, you must run the magnetic pole detection routine (parameter PA03) before the motor will track properly. The manual buries this in chapter 11 and it is easy to miss.