I found your post researching what seems to be a very similar issue. During the "event", a stepper makes an alarming high pitched noise. Large numbers of steps are lost. An axis often moves at a slower rate during this. No mechanical binding ever found afterwards. Re-home the machine and it might run perfectly the next time.

We share a very similar sounding setup. UCCNC running through UC400ETH. Same manf VFD if not exact model.

Last evening it occurred again several times. I have not checked for stray AC voltage but will do that today.