One of the disadvantages of a front-loader is that the door seals get wet, then you leave the door closed in between uses. They may never dry out. Depending on the design, it could be an accumulation of soap scum and maybe lint, hair, etc. You might try an empty load with a high concentration of bleach and see what happens. Maybe stop is part way in between to let things soak, the resume. After it completes, try leaving the door open at least overnight so it can dry out well.