No valid JSON content returned from Api Server

This error indicates that you need to updater your server software.

Our certificate is fully valid and working correctly. But providers for SSL certificates introduced additional changes, that could lead to old software generating this warning.

More information  - https://letsencrypt.org/docs/dst-root-ca-x3-expiration-september-2021/

DST Root CA X3 will expire on September 30, 2021. That means those older devices that don’t trust ISRG Root X1 will start getting certificate warnings when visiting sites that use Let’s Encrypt certificates. There’s one important exception: older Android devices that don’t trust ISRG Root X1 will continue to work with Let’s Encrypt, thanks to a special cross-sign from DST Root CA X3 that extends past that root’s expiration. This exception only works for Android.