@Steve-Pippin i had exactly same problem on my server just other day, try many things but nothing worked but followed this and instantly it stopped giving me and my users this exact same error you are getting.
Filled in Custom Certificate Domain with a DDNS address i have like this guy did.
if you dont have a dynamic dns service just put in your wan ip if its static and doesnt change after few days/weeks or you’ll need a DDNS running on server or your router.
@jackmeat give this a go also, solved the problem for some of us anyway if you further below in that topic
