I have two identical servers that suddenly stopped auto-renewing wildcard certificates from LetsEncrypt somewhere back in March or April. (probably earlier, but that's when they started expiring). I can manually renew them, and have had to do that twice since then. But they still do not auto-renew.
The best trouble-shooting I've been able to accomplish was that LetsDebug gave me errors connecting to the website when the website had both IPv4 and IPv6. Removing IPv6 (AAAA) DNS records immediately cleared up the LetsDebug errors and the manual renewals proceeded. I'm waiting to see if that fixes the auto-renew, but I won't know until the first week of October (30 days from expiration).
But its a bit counter-productive to have to delete AAAA records from sites.
Plesk is fully updated (18.0.63 Update #1) on both servers, running Debian 10.13 (1&1/IONOS repo). Above errors were present after the last Plesk update.