For addon users:
By default, addons with a mismatched API Version field will not be loaded. You can force the game to load these addons by pressing Esc, going to Addons, and clicking the "Load out of date Addons" checkbox. This will allow the game to load older addons, but also means that some of the addons loaded may break (since the API they use may have changed).
You'll need to do this until all of the addon authors release updated versions of their addons with the new API Version number, thus indicating that they know the addon works with the latest version of the API.