Sounds like power block (IGBT Modules) issues or possibly the driver modules (1DD/1DT) to me.
Bugger to test out unless you have spares though.
Motor power will always go off as it is classed as a fatal error. and any power block related issues usually will not show their face until actual motor power is applied.
So you are unlikely to see it during startup, only when demanding motion.
Have you checked any of the documentation here specifically the AD Electrical Manual for further assistance and checks, it contains a lot of good detailed information that may help.