What you see in the log is the Wave Server letting you know that
something went wrong. The issue with the current Wave-Robot-API
library is that it is not able to decode the error, yielding to a
critical error in the log. This is what you see.
To get more information, you must enable the robot to log at a FINEST
level, and then look in the logs for more information about the error.
This is discussed here:
http://www.bitsbythepound.com/debugging-a-google-wave-robot-by-inspecting-incoming-messages-291.html
Finally, a side discussion on the OPERATION_ERROR is discussed here:
http://www.bitsbythepound.com/how-to-change-the-state-of-a-gadget-using-a-google-wave-robot-293.html