On 30th May 2020 the Sectigo AddTrust External CA Root certificate expired. Previously they assured everyone that no issues will be. However, the reality is that some legacy servers/devices are affected.
That is a ridiculous situation. We pointed their attention to the expiring AddTrust RSA/ECC multiple times within a year and each time Sectigo assured us no issues will be.
Please note, the SSL certificates itself are active and modern devices and browsers do not show and would not show any error. All SSL from 30th April 2020 issued with the latest files valid till 2038.
What to do if you faced any issues?
Remove the old expired AddTrust RSA/ECC file from your server and replace it with the latest one. You can download it simply re-download the latest files from Client area / API in a ZIP file or CRT format.