Can anyone suggest a reason why Undo might disable DocumentCollection reactors?
It has something to do with me undoing the annotation scale as when I remove the line
db.Cannoscale = tstAnnoScale
The problem doesn't occur.
Hard to say without seeing code. Event handlers can become disabled if you allow an exception thrown in your event handler to propagate to AutoCAD (i.e. if you don't catch and handle the exception in your event handler).
I've only just fixed the problem but I'm not entirely sure where the exact problem was as I changed a few things and moved a few buits of code around.
One thing I found was that two events were bouncing off each other but for some reason it wasn't and endless loop.