The problem, as described in the original (archived) report, occurs only when
deleting the root node, otherwise the undo functions properly.
delete root node (accidentally or not)
hit undo to restore it
After deleting, undo button has no effect at all.
Also, it doesn't grey out after exhausting the number or undo levels available.
Tested on a new document, where I did not have more than 10-11 steps to undo,
so it seems to be stuck there after the mentioned deletion.
The problem becomes even more irritating if one has autosave enabled (as it is
by default) and after deleting the root node the document is lost forever.