Postgres unsupported frontend protocol

I have just set up Drupal8 to work with Postgresql 9.6 and have been going through some tutorials. Not sure if that is related, but I have recently seen the following Postgres error:-

postgres[47542]: [3-1]: FATAL: unsupported frontend protocol 65363.19778:
server supports 1.0 to 3.0

Anyone any idea what any of this means?

I am running FreeBSD 10.1 amd64 on a VPS.