I noticed leak from my cellar wall. After digging a bit I managed to find leak coming from back of the compression nut of the elbow connector that connects to the supply pipe. What could be the reason? There is not enough wriggle room to tighten or remove the connector.
Problem with the connector. Either faulty/failed it not assembled correctly. You're going to have to chisel out to be able to get some wiggle room and then turn off the water outside then replace the elbow connector. Inspect before you fit the new one to make sure there isn't damage to the pipe, and make sure there was an insert present in both pipes.