First of all could you please check whether the new connection’s server is working fine or not i mean the connection that we define in the datastore(oracle,sql or etc:-) sometimes the server might be down(oracle,sql etc). due to this you are unable to fetch the data i guess.
Also, could you please check how many records that you are pulling or loading into the new connection.
Also you said that the new connection is working fine with some test job. can you tell me hoe many records ou are pulling or loading in the new connection in your test job.