[imp] Failure writing session files?

[Chris Hastie]

I've suddenly started getting messages like this when trying to login 
(HEAD branch):

Warning: Failed to write session data (files). Please verify that the 
current setting of session.save_path is correct (/tmp) in Unknown on 
line 0

Trying again leads to

Warning: Session object destruction failed in 
/usr/local/www/mail/data/horde/lib/Horde.php on line 148

and a repeat of the first warning. It started a few days ago after a 
server reboot, but seemed to settle down. But now it's back with a 
vengeance and I can't log in at all. Any pointers? I've tried making 
local changes to session.save_path (using a php_value directive in a 
.htaccess file) but get the same message whatever session.save_path 
value I have set.
-- 
Chris Hastie