Information on this can be found on our FAQ here:
Here is what it says to save you looking:
This error occurs because the web server that WordPress is running on was unable to validate our backup vault server. Security features in place mean that if the web server is not sure the backup vault server it is talking to is actually Backup Technology's backup server, it will not send the backup data.
So far, we have discovered two reasons why this error may occur. The first is that somebody is trying to intercept the backup data, and is pretending to be our backup server. The other is that the web server is extremely out-of-date and does not have the means to validate server identities - the information required to do this changes gradually over a period of years and if a server does not update this information for several years it can cause problems.
In both cases, you should contact your web host for help with this, as the error is returned from their server when we attempt to contact https://wordpress.backup-technology.com/ from within the plugin.
However, in the latter case, it has been known for web hosts to be unhelpful, unwilling or just unaware of the issue and not fix it. Some users have switched providers in this instance. If you are still stuck with the issue and cannot switch, feel free to contact us through our support channels (visit our website for details.)
If you absolutely need to disable this security, you can use the following instructions. But please do remember that this makes the backup communication insecure and vulnerable to man-in-the-middle attacks - and just remember even if you have your backups encrypted before transmission - it is still less secure than if the server problem is fixed so that it can establish secure transfer.
Please go to General Settings in the plugin.
Then in the address bar of your browser, edit the address so that it ends in "section=advanced" instead of "section=settings".
This will open the advanced settings page - best to leave everything here alone as the plugin handles it all and it is just for fixing rare issues. Actually modifying things here can break things.
On here you see at the bottom the option to disable SSL verification, ignore_ssl_cert. You will need to TICK the box next to this option, and then choose Yes. The checkbox enables the override and then you choose what to override it with.
Hope this helps,