I run over a dozen sites all with SSL supplied by LetsEncrypt, which I think is what Nabble uses.
I may misremember but recall that, LetsEncrypt issue certificates that are valid for three months and as they do so they plant a set of files on your server. Then, every two months, their bot checks those files exist and issues a new certificate.
Perhaps once in 12-18 months, for what ever reason, probably because their bot tries to find those while a backup or other process is running at my host, I get an email saying the new certificate was not issued and I have to manually get LetsEncrypt to re-issue the certificate.
I suspect something just happened to be running at Nabble when LetsEncrypt checked for the presence of the files this time.
Volunteer Helper - but recommending that users move off the platform!
Once the admin for GregHelp now deleted.