I have had my very expensive Rosieres oven just 7 years and today whilst cleaning we found huge holes along the back of the floor when spot welds have turnrd to rust. This is just as cars used to go. Having contacted Rosieres they are not interested in any help, compensation etc. Be warned as they are supposed to be a top French brand.
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Rust on the bottom of the cavities are where they have been cleaned then not dried fully. If you clean the oven always turn the oven back on for about 5-10 minutes on full so that the heat dries out the damp that you an either not reach or see.
The problem you have happens on ALL ovens if they are not dried out fully.