Can't request letsencrypt certificates

Operating system: CentOS
OS version: 6.9

All of a sudden I am not able to run letsencrypt certificates. I get the following error in the web gui:

Failed to request certificate : Failed to lock file /var/webmin/modules/virtual-server/letsencrypt-lock after 5 minutes. Last error was : Locked by PID 19105

Then, in the messages log, I get the following errors:

Feb 5 07:28:15 web01-sfo named[31650]: client @0x7f34300a9060 127.0.0.1#55968 (acme-v02.api.letsencrypt.org): query (cache) ‘acme-v02.api.letsencrypt.org/A/IN’ denied
Feb 5 07:28:15 web01-sfo named[31650]: client @0x7f34300a9060 127.0.0.1#55968 (acme-v02.api.letsencrypt.org): query (cache) ‘acme-v02.api.letsencrypt.org/AAAA/IN’ denied

Nothing has changed on the system, and has been solid for several months.

Any idea on how to fix this?

Have you request more than 50 this week? if so, you have passed the limit for more info: Rate Limits - Let's Encrypt - Free SSL/TLS Certificates

If let’s encrypt is not working, you could still try Cloudflare OV Free SSL.