I think this problem is totally caused by Liquid detergents which don't have bleach as an ingredient. I no longer use them and the problem of smells and black mould has gone. I don't need to do a service wash and only very occasionally use 60 degree wash.
I did initially run two 95 degree washes using detergent then distilled vinegar and scraped round all the door seals several times to get rid of all the brown gunk.