I see. disroot.org is indeed using a Let's Encrypt certificate. It's configured properly and it's chained up to the DST Root CA X3 root certificate, which should enable secure connections most devices, even older ones.
outlook.com and vivaldi.org don't use Let's Encrypt certificates. Do you get the error on those domains too? Or only when using disroot.org?
It could be your Epson printer doesn't even trust the widely trusted DST Root CA X3 root certificate, which could lead to this error. Your error might go away if you add the DST Root CA X3 root certificate as a trusted root certificate to your Epson printer. You can find the root certificate on the Chain of Trust - Let's Encrypt documentation page.