The error is caused due to overlap of connectors or instance, if you see an error as below:

And you can see Multiple clusters with same name, which is caused due to running the notebook_1 under a cluster attached to it and re-running a notebook_2 with %run / sub notebook run, which is connect to the same notebook_1 cluster. Which cause below scenario: Same cluster under two resource. 

Solution:
Detach the cluster from notebook_1 & notebook_2,

Now refresh the web page and re-attach the cluster to notebook_2, run individual cells one by one not RUN_ALL at a time. Once you've run two to three cells successfully. Now Run All and test, it will work as below.