Yeah the certs expired, I think HT is using those crappy Lets Encrypt free certs which expire every 90 days. A proper cert only cost a couple of $ or a wildcard cert would cost about $50.
Technically, there's nothing wrong with Let's Encrypt certificates and Let's Encrypt provides wildcard certs too. They're trusted by all major cert authorities and are even cross-signed by IdenTrust for very old clients.
If short duration is a problem, you can automate renewals with CertBot (or any other ACME client).
No need to waste $$, unless you need OV and/or EV certs.