Problem "forking" site: getting IndexExceptions

Hiya, folks --

The site I'm managing is undergoing a major revision, which will take
a while to complete. I want to fork the content: the old site will be
running as usual, while the new site is being worked on. So, I made a
copy of the old publication with a new name, and revised
config/publication.xml to match.

Now, everything else appears to work, except that I'm getting
IndexExceptions e.g. when deactivating a page. These are caused by the
Lucene module (I can produce a whole pile of them by attempting to
re-index it). I figure that the change of URL -- or something like it
-- must've screwed up something. How do I get rid of them?

Tried with Lenya 2.0.3 and 2.0.4, behavior is identical.

TIA, Petteri