The grill function of the Severin MW 7875 is not designed as a bottom or top heat, as is the case with conventional ovens or grills. Instead, the appliance has a separate grill unit with an output of 900 watts, which allows food to be cooked and browned. The grill function can be used in combination with the microwave function to achieve even heating and browning.
Whilst you can cook and brown food with it, it is not ideal for toasting bread in the traditional sense as it does not provide specific top or bottom heat like a toaster or oven.
I suspect a defect in the magnetron (glow cathode, electromagnet) If a sausage does not burst at 900W within 60s, something is wrong. I would send it in for warranty.