My apologies, I probably need to make this clearer in the documentation:
Web Display checks that the server and template versions are compatible when it starts up.
This means that the major and minor version numbers must match, so template 2.3.1 will only work with a server version of 2.3.x, and not 2.1.0.
This is done because some changes made to Web Display affect the data that the server provides to the web page, so they must be kept in step.
In practice, any server version a.b.x will work with any template version a.b.y, and updates are normally provided together.
Cheers,
Martin
(Web Display developer)