@Basketkase welcome to the forum.
I believe that is your problem. Try a current release. :latest is a v2 server and so requires a v2.x client.
Also latest is not actually being fully used for the server image yet as it is still beta software. The tag is therefore referencing an old beta version. If you want to try the latest server version you will need to use joplin/server:2.2.7-beta