Optional arguments are not supported
The ProviderViewHelper registers all controller actions which are annotated with @extdirect. For correct handling within ExtJS, the number of arguments for each action must be registered as well.
Now the problem is that the ProviderViewHelper just counts the number of arguments of a PHP method and uses this number as an argument count for ExtJS. If, however, an argument is optional and the client side doesn't define that argument, the communication between client and server through ExtDirect fails.
The solution, for now, is to just ignore optional arguments.