Also note that server sends the "short chain" so will not be supported on older Android devices (pre 7.1) or other systems that do not have ISRG Root X1 in the trust store.
The post did not mention needing to support older devices so perhaps this is not a problem. I just thought it worth mentioning since it is questioning browser support.
Maybe see below topics if the HTTPS url is the one that is failing. But, I agree you should ensure they are not using an HTTP url first.