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The current site certificate looks like it was just issued yesterday (June 15) for a year, it comes from Amazon and the Amazon intermediate certificate ("Amazon RSA 2048 M03") was only created in August 2022, although the Amazon root certificate ("Amazon Root CA 1") dates back to 2015.

What brand of tablet, and what OS and version, were involved? Browsers will have a trusted set of certificates that they either provide or obtain from the OS, and if they don't have something that trusts the issuer of a site's certificate, it will give that warning. And if the tablet is old enough not to be getting any sort of software updates, it may not be getting newer root certificates to trust and might throw this message. That Amazon 2015 root certificate should be in even fairly old tablets, though.

Peter, who was issuing your site certificate before you switched to Amazon? If the error isn't happening anymore, it may have been that provider that was issuing off something that wasn't in the tablet's trusted certificate bundle.

Yikes, Android 5 "Lollipop" was the 2014 release, and 5.1.1 seems to be from 2016. Android tablets were notoriously poorly supported, but that's really sad. If the browser app was being updated after that, you may get some newer certificates or security patches that way.

I think they've been trying harder again on Android tablets, but I'm pretty sure Apple still has well over half the tablet market.

Ah yes, Let's Encrypt - we use that at work for a public server I write an app for, although I managed to pass off the certificate renewal worries to our IT folks. I remember seeing something about their expiring cross-cert signing thing awhile back.

Bill, as mentioned in Peter's link, if you are able to get the Firefox Mobile app on your tablet, it apparently does its own trusted certificate management, so if you run into other browsing sites with the same problem using Chrome or whatever you do, Firefox may have newer trust for them and not put up the warning.

You may start coming across more - Let's Encrypt provides free certificates that a lot of smaller sites use instead of having to pay for a commercial certificate. Let's Encrypt certificates are only valid for 90 days, so as those get renewed they'll be based off LE's own certificate that the old Android versions don't know about. Sounds like Firefox keeps its own trust bundle, and the app may still be updating that even if Android has long since stopped updating its own bundle that Chrome would probably be relying on.