To fix this problem, the NFS file handles must be renegotiated.
The Python errors (Traceback) is because the NFS location was not available when it was trying to either read/write to that location.
When you restarted and no errors were reported indicates that you are fine. The NFS mnt point was re-established.
You can also try:
Try one of these on the client machine:
a) Unmount and remount the file system, may need to use the -O (overlay option) of mount.
From the man pages:
-O Overlay mount. Allow the file system to be
mounted over an existing mount point, making
the underlying file system inaccessible. If a
mount is attempted on a pre-existing mount point
without setting this flag, the mount will fail,
producing the error "device busy".
b) Kill or restart the process trying to use the nonexistent files.
c) Create another mount point and access the files from the new mount point.