Failed servo/ encoder help

[Bongo]

Hi Friends. Struggling with fixing the machine I built a few years back. It has a twin rotating ball nut design on either side of the moving gantry. here's the machine build video : https://youtu.be/cqYJS27aC4w?si=ir_XA7-AvJQFJMMD

One of the 400W servos that drive the gantry is playing games. I have been round the houses tracking down the problem (different sides of the gantry moving different amounts, is scary!). At first I thought I had dirt binding the ballscrews but not so. I only found the suspect servo after getting two dial indicators on either side of the gantry. I can see one side gets consistently out of true - eventually faulting the servo drive as it attempts to rack the gantry relative to the side that stays true... I have tried a different servo drive (no change) different cables (no change), switching the output pins to eliminate software and cables from the computer to servo drives... etc etc..

In fact, in 'smart jog' (the servo tuning software, I can see the servo's mechanical angle changing as the faulty servo returns to what should be 'zero' after moving about, while the recorded position feedback stays the same. I think it's therefore probably a problem with the encoder that looks like it screws off the back of the servo...

It's a long shot I know, but has anyone got one of these servos cluttering up a draw somewhere?? It's a CSMT-04BB1ANT3

A more reasonable question- can anyone offer guidance on the repairability of the encoder and any pitfalls to watch out for when dismantling?