Search Form returns 'page not found'

EDIT: Apologies - submitted as bug report when should be a support request...

I'm using Advanced Forum along with Pathauto to change URL aliases, and Page Manager to customise the layout.

I've enabled the Forum 'override' in Page Manager, and added content 'Search Form' from the 'Forum' section to this page's content.

The search box appears on the Forum page, but when I search I get a 'Page not found' (404) for http://www.example.com/forum/search

I've tried clearing the caches, and uninstalling/reinstalling this module, but it makes no difference. any clues as to what's going wrong?