OK, this was due to my manual hacking of the XML - I had added a Formatter entry and pointed to an ExceptionFormatter. My mistake.
Then the next error was:
"Unable to handle exception: 'LoggingExceptionHandler'."
Ah, come on... anyways, good ol' google to the rescue and I found this thread:
"must expose a public constructor that takes a TextWriter object, an Exception object
and a GUID instance as parameters."
I then changed the EventTopicExceptionFormatter and added the following constructor:
public EventTopicExceptionFormatter(TextWriter writer,
Exception exception, Guid handlingInstanceId)
: base(writer, exception, handlingInstanceId)
I hope this helps someone out if they come across the same problems.