A soft 404 is a page that looks like an error page or redirects irrelevantly but technically sends status code 200. Search engines may wrongly treat it as a valid page.
A occurs when a non-existent or empty page looks like an error page but technically returns status code instead of sending a real 404.
Example: the URL /alte-seite shows the text "This page was not found" but responds with status 200. Search engines may interpret such a page as a valid page.
Common causes are:
redirecting all errors to the homepage,
error pages that send status 200,
empty product pages or internal search pages with no results,
very with hardly any content.
The fix: genuine error pages should send (or 410 for permanent removal). A 404 page may be designed and helpful, but must technically remain a real 404. Irrelevant to the homepage should be avoided.