500W Inverter + Laser Printer
you can try and measure the peak current by monitoring the current while the printer is connected to the mains power,(caution about 230Vac). As there won't be a breaker dropping out you should be able to see the current drawn through the complete cycle of the printer.

However I caution you about measuring current on the 230Vac.

It does sound like the peak load at warm up is outside the peak that your inverter can provide.

The heater that is being turned on would have a low cold resistance and hence draw more than its running current when first switched on.