Virtual Programming Lab (plugin)

Taking into account the information provided, It seems that you are using an intermediate Certification Authority and the browsers do not have it. If this is the problem then you must provide the certificate of the intermediate certification authority to the browser.

I think you must concatenate the intermediate cert authority certificate to your certificate.
You may use a command like:
sudo cat Intermediate_CA.pem >> /etc/vpl/cert.pem
sudo systemclt restart vpl-jail-system
Notice that the pem format is plain text.

Best regards,
Juan Carlos.