Overwatering is much more likely to occur when a plant is moved unnecessarily into a larger pot. The added soil tends to hold on to water for too long and suffocate the roots. In general, it is best to leave a plant in the pot that it comes in.
Replacing and/or changing potting soil often damages the fine root hairs that do most of the work. I suspect that is what you are dealing with here.
Keep your Rubber Plant close to and in front of a sunny window. Allow the top inch of soil to dry before adding just enough water so that the top inch of soil dries out again in about a week. Experiment to determine what the right amount of water is.
When plants are kept in their nursery pots, they are much easier to water and much less prone to root rot.
