Thanks for having a look. Since raising this bug I upgraded to PHP5.3 
(from 5.2) - the issue seems to have gone away now that I set logging 
to DEBUG and enabled the SQL based sessions again.

Can't reproduce on my setup - can be closed from my perspective.

Reanalyzing this - this is not an issue with Horde.  Most likely the 
DB driver is failing when closing the original session, which is 
causing session_close to fail, so reopening the session is also 
failing (open() in the SQL session driver does nothing, so that can't 
be failing).  You need to look at the HORDE log, not the PHP log, for 
further debugging details.

Dynamic IMP view is special in that it will close an active session 
and reopen it in the same page access in certain instances in order to 
prevent session deadlocks for other simultaneous accesses to the 
server.  So this is not an issue with IMP.

If anything, there is a probably an issue with the way we are closing 
the DB connection when destroying the session that causes the attempt 
to reinitialize the session to fail - this is almost certainly an 
issue within the SQL session driver.

When using the SQL Database session handler (and mysqli driver) I get 
the following errors from DIMP and it fails to load any emails;

PHP Fatal error:  session_start(): Failed to initialize storage 
module: user (path: /tmp) in 
/var[...]/horde4/imp/lib/Ajax/Application.php on line 456
Jul  2 07:22:01 skadi HORDE4: PHP Fatal error:  session_start(): 
Failed to initialize storage module: user (path: /tmp) in 
/var[...]/horde4/imp/lib/Ajax/Application.php on line 685

Other modules (turba/kronolith) work fine with the SQL based session...

If I choose to login to the 'Traditional' interface it works fine, so 
this seems isolated to the DIMP client.

Changing the Session handler in the Horde settings to 'Use the default 
PHP session handler' which is indeed file based in /tmp everything 
works as expected.