The issue might be already fixed, but I would leave a comment to this issue.

I encountered the same issue at the clone task execution.

I check the log message at the client node which ipynb is executed, the WARNING message was displayed "ClearML Could not read Jupyter Notebook: No template sub-directory with name 'script' found in the following paths ..." during the task creating.

I installed 'nbconvert' in client node and then issue was gone.
It might be better to install 'nbconvert' if you will entry tasks from jupyter notebook.