I have no idea why, but my microwave stinks. Smells like something has crawled into it, died and then decayed slowly. Scrubbed it, used kitchen spray on it, used dettol spray on it, left a tub of bicarb in it and it still smells.
Lol Barmaid@13:58. Strangely enough Barmaid when I posted earlier I was actually thinking of the Brother combi microwaves that had an interior base that had a seal failure and gave the problem you describe. I don't actually know the Kupperbusch oven but it is worth you having a close look around the three edges of the oven bottom to there is indeed a seal. You may not be able to see a seal but it may be just like a rubbery filling around the edges and they break down in the corners. Not really a diy job I am afraid.
I was reading this at work today and as there is a rather manky microwave in the staff kitchen I thought I'd give it a go. Didn't have a lemon but did have a left over mandarin from a business buffet. Chopped it up, put it in a mug with some water and set microwave for 2 minutes. My god it made a mess :-)
I squeeze a lemon in water and microwave it including the lemon for about 15 mins it completely neutralises it and the steam loosens any bits you just wipe off. I do it once per month