What's wrong with my rubber tree?

I urgently need advice about my rubber tree/ficus. It's about 3 years old. Dark spots are spreading from the bottom of the leaves. It's not mites, at least I've been told. The leaves turn yellow/brown at those spots and then fall off. This only happens to the lowest leaves. The top shoots are healthy. I water every 1-2 weeks; the soil is more dry than moist. The plant stays in the same place by the window all year round.

What can I do? I'm very grateful for any advice and hope the plant doesn't end up bare!