Jupyter notebook: interact widgets fail to operate correctly

I was running a simple sine wave generator program using widgets with interact command on a Mint 18.3. Perfect! The program commences with

Blockquote
%matplotlib inline

import matplotlib.pyplot as plt
import numpy as np
from ipywidgets import interact

Blockquote

Now I have installed a fresh Mint 20, and Jupyter Notebook, Using the same program I get the following message:

Blockquote
Widget Javascript not detected. It may not be installed or enabled properly.
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When I operate the widgets, I get:

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The save operation succeeded, but the notebook does not appear to be valid. The validation error was:

Notebook validation failed: {‘model_id’: ‘7196c42ed6f441279205c0eb24207f99’} is not valid under any of the given schemas:
{
“model_id”: “7196c42ed6f441279205c0eb24207f99”
}

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Can you help me to get this set up again correctly, please.

Thanks,
Robin

SOLVED. Total re-install and then works correctly.
Robin