Single site disconnected; throwing error on reconnect attempts

Just one of my sites (schoolsays.co.uk) has disconnected from the mainwp dashboard. When I try to reconnect I get this error:

ERROR: HTTP error - error:140770FC:SSL routines:SSL23_GET_SERVER_HELLO:unknown protocol

The only change that has recently been made to the site is that a quad A record (AAAA) with an IPv6 address was added to the domain’s DNS.

Would greatly appreciate any advice on what the problem might be.

Thanks,

Chris