...the part that you cut away, that was fiberglass and resin. What you put back on, that was just resin. So the problem is, that's got no strength anymore...
2nd problem...if you put a polyester resin on an EPS (epoxy) foam board, as I understand it, it can damage the EPS foam.
Yes... and yes. If any poly resin got down under that layer of epoxy and glass, the styrene in the resin will literally dissolve the polystyrene (EPS) foam... if it is, indeed, polystyrene foam. There's always the possibility of epoxy being used over polyurethane foam, which of course is fine.
After looking at the pictures, there's a lot of issues with this board, including water damage along the stringer. Looks like mold growing in the foam, which is pretty common with old, waterlogged boards. I wouldn't invest much into repairing it, except for sanding down that resin you added, and putting a layer or two of cloth over the area. Then sand that down, and paint some resin over that. Sand it smooth and ride it.