@tspence as @jiyunson said, a screenshot would be helpful. Another very helpful piece of information would be the exact URL the student is using when they get the error. This will help me look up the error and get an idea of what is causing it.
In most cases, a 405 error is caused when the student is logged out of the JupyterHub and then tries to access a notebook without going through Canvas. Usually if they go back to Canvas and click to open the notebook again, it should work out.