This can be a common issue:
First let me explain what it means: The message "server state differs from client" simply means that the server and your browser (client) do not agree on the location of troops.
You may be able to resolve this by closing the game window, clearing the browser cache, then closing and restarting the browser. This works at least half of the time.
If this doesn't work you will need to fill out a bug report from your game, and a game operator can refresh the server, which will reset the units. Click the menu in the lower right corner of the game, and click the "bug" button for bug report.
