And.. I found that the timing belt for AZ encoder became sticky and some teeth are now missing.. the rubber deteriorated and soon it will turn into a paste-like goo.. so it's time to change timing belt (420 mm, pitch 2.5 mm, 168 teeth).
I found it on AliExpress (10 pcs for ~$35, free delivery) and RS-components (1 pcs for ~$35, belt + postage).
Also
on MBA for 7.66 AUD (+ postage and GST)