Error when batch updating content

When trying to perform the task "demote from front page" on all but three nodes (as selected), I receive this error:

The update has encountered an error.
Please continue to the error page
An HTTP error 404 occurred. /batch?id=4&op=do

(continue links to "/batch?id=4&op=finished" -- the link goes to a blank page)

What's the problem? I'd like to perform batch operations, of course..