I was going to power on the server remotely to do some work today and noticed that the web gui wouldn’t work. (Chrome said “server error” and claimed the certificate had expired about 4 years ago  (this explains why the browser has been refusing to load the web gui every now and then for the last year or so)).
There are several ways to solve this.

One solution is to power off the server, pull the power for a few seconds to reset to drac, another is to ssh to it and type racadm reset (which reboots the drac), to reload the certificate.
it took some digging, but eventually I found an article dealing with this problem on a more permanent basis. (rebooting the DRAC or powering off the server is not always very practical.)
1. SSH to the DRAC.
2. run the following command: “racadm sslresetcfg” to create a new SSL certificate.
And voila! my certificate is valid till 2030:)
problem solved for the next 26 years.