I have now learned that my employer uses a Palo Alto firewall, and I suspect we are running into this issue:
However they are saying the acme-protocol IS enabled on the firewall.
They are asking is there any chance the request is coming from overseas, since they block a lot of non-US countries.
And also, is it possible to find out the address from which the request is coming, so they can check their logs.
Thanks a lot for your help.