This is confusing to say the least...!
If I disable the firewall for the server and for plesk, the connection problem persists.
I have restarted the server and all services, but the connection problem persists...!
No matter what changes I make, with the firewall on or off, the problem is always the same. Renewals are ONLY possible if the HTTPS redirect is switched off first.
It is persistent and applies to every domain, whether they are WordPress or not.
It happens with the firewalls off as well.
As I have said previously, this has only started happening in recent weeks. Everything worked perfectly OK before - for ages.
Plesk has been updated, so has the Let's Encrypt extension. Could there be something going on there? But if that were the case I wouldn't be the only person with a problem...!
My detective work continues...