But I found the error and commented it below in the Community Content-section. For completeness I'll post it in my blog again:
There exists no button with the ID 'Ribbon.Library.Actions.ConnectToClient.ReplacementButton' so a javascript-error is thrown in the browser. The part of the xml must changed to this line and then it works:
<CommandUIDefinition Location="Ribbon.Library.Actions.ConnectToClient">
<Button Id="Ribbon.Library.Actions.ConnectToClient" Command="ReplacementButtonCommand" ...>
Just remove the string '.ReplacementButton' in the Id of the Button from the example and it works.