After upgrading to 4.2 we had problems deleting emails, from dimp there was no problem, but imp would complain that the link you tried was only valid for 0 seconds.
The fix on this page solved that:
http://lists.horde.org/archives/imp/Week-of-Mon-20080526/049140.html
Mainly, put these lines in horde/imp/config/conf.php:
$conf['server']['cache_folders'] = true;
$conf['server']['token_lifetime'] = 1800;
$conf['server']['cachejs'] = 'none';
$conf['server']['cachecss'] = 'none';
Additionally, we changed our tokens in horde from sql to file.
