That sounds like a cover plate for either a drum trap or cleanout plug. You apparently have 2 problems. One is a blockage. The other is the cleanout or drum trap plug is leaking. It might be fixed by cleaning out the trap or using the cleanout to clean out the drain line and replacing the plug/cap. But at that age the trap or drain might be "worn" so much as some of the drain system will need to be replaced.
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