Thank you for the advice! Lowering the voltage to 40V has improved the situation drastically.
There is however another question which has come up: As the TMCM-142 offers quite some power, I'm wondering if there's a chance to enable/disable the motor (or at least motion) by a hardware signal other than turning off power and subsequently losing communication and all parameters?
Or, the other way round, is there a way to keep the ARM7 processor and the TMC457 running without motor power? Supplying +5V via the CN1:10 pin (+5V out, yes, I know this is weird) sort of does the job but prevents powering up correctly once the external supply is back.