The Invalid Error Handler error is issued in the following situations:
More than one error handler for <exception> exception in flow: you are catching the same exception in more than one error handler in your flow.
Remove the duplicated error handlers for the same exception.
'Destination' screen of 'Security' error handler must have 'Anonymous' role set in <flow>: you have an Error Handler to catch security exceptions whose destination is non-anonymous screen. The exception flow of an error handler that catches Security or Not Registered exceptions must start with an Anonymous screen, otherwise it won't be possible to execute this exception flow.
Depending on your application requirements, you have to do one of the following: either change the destination of your security error handler to an Anonymous screen; or change the role of that screen.
<exception> error handler cannot be connected to a screen with mandatory parameter(s) in <flow>: you have an error handler linked to a screen with mandatory parameters and this situation is not allowed: when the exception occurs it won't be possible to send values to these parameters.
Depending on your application requirements, you have to do one of the following: either change the Is Mandatory property of the screen input parameter; or change the destination of the error handler to a different screen with optional parameters.
'Exception' must be set: you have an error handler that is not catching none exceptions.
Edit this error handler and set the suitable value for the Exception property.
Clicking the symbol will take you directly to the problem handler.
Handle Exceptions | About Error Messages