By default mastercore/bitcoin only accept 4 active rpc connections.
We where observing some instances where these 4 connections where getting locked/blocked by process not ending appropriately. We've increased the number of active connections for mastercore to remove the back-end limitation but this issue is being opened to track/investigate to make sure that the rpc connections for broadcasts coming from omni are properly being handled/closed once completed or timing out properly if the page is abandoned.