First your buss voltage is way too low
For a 400V input it should be about 687 VDC and for 480v input it should be 825 VDC with no load
What you describe tells me that you have a single phase condition possibly a blown power diode to DC buss
” This drive is connected to a motor with a nameplate that gives both 400V / 50 cycle and 460V/60 cycle nameplate values. (An SEW gearmotor).”
That tells me that it is a 4 pole motor, base speed at 50Hz would be 1500RPM and at 60Hz it would be 1800RPM
From your description they want to run it in over speed 70 – 90Hz range, that is doable and is done all the time. As we all know no standard motor will run at it’s base speed they all have to slip to develop torque so the normal rated speed for a 4 pole 60hz motor is about 1750RPM. I have run 4 pole (Base Speed 1800RPM at 120Hz many times without any problems
As for de-rating the drive should not be necessary
Keep in mind that with a standard motor if it’s running below base speed it is running in constant torque variable HP mode and above base speed it is running in constant HP and variable torque mode so the drive should not need to be rerated. Below base speed / frequency the vfd will supply a constant volts to hertz ratio. 8volts / hertz, above base speed the supplied output volts to hertz ratio is reduced The voltage to the motor remains constant while the output frequency goes up.
A motor running at synchronous speed develops 0 torque, that’s why the label always show a speed less then synchronous speed it call slip. Example a nameplate rating of 1750 RPM is a 4 pole motor base speed of 1800 RPM the motor is fully loaded when the RPM’S slip down to 1750
As you can see don’t really don’t need to reduce the supply voltage to 400
The motor has is rated 400V 50hz / 480V 60Hz in the US the normal line voltage is 480V 60HZ
With running the motor at the higher frequency / speed you would be better to use the standard 480V supply this would give you more voltage available for the higher frequency you want to run at.
I would recommend that you replace the vfd with a Yaskawa brand I am sure it will give you much better performance then what you are using.
Let us know what you find