The operating system my web server runs on is (include version):
Ubuntu 16.04.2 LTS
I can login to a root shell on my machine (yes or no, or I don’t know):
yep
I just received an email saying that my ganchrow.com certificate will expire tomorrow, but all signs seem to indicate that the certificate will expire on August 24 and is therefore not currently up for renewal. For example, if you open up a browser at https://ganchrow.com, and check the certificate, you can see that it is valid until end of August.
I am confused. Can someone describe what might be going on here? Thanks for your help.
The warning emails spell out exactly which set of names was on the certificate which is due to expire. You will find that the certificate you’re actually using has a different set of names in it. The Let’s Encrypt systems assume that since you had certificates separately with and without the extra names you must want both and need to be warned that one is expiring without being renewed.
Specifically in your case you had a certificate issued for ganchrow.com, and then later you had one issued for www.ganchrow.com AND just ganchrow.com. I believe it is the former which is expiring without being renewed.
Very likely you didn’t actually need a separate certificate anywhere just for ganchrow.com, and you’re happy to keep using the one with both names in it, so you need do nothing, but the warning system doesn’t know that.