[horde] Memcache issue

[Administrator BCS]

Got Horde is up and running but performance is not what it should be.  
It seems I have a problem with memcache. Memcache is up and running, I  
can connect to it with telnet localhost 11211, but the stats tells me  
nothing is happening there.

When I enable Memcache server under Configuration>Horde>Distributed  
Hash Table, my conf.php contains:

$conf['sessionhandler']['type'] = 'Builtin';
$conf['sessionhandler']['hashtable'] = true;
...
$conf['hashtable']['params']['hostspec'] = array('localhost');
$conf['hashtable']['params']['port'] = array('11211');
$conf['hashtable']['params']['weight'] = array();
$conf['hashtable']['params']['persistent'] = false;
$conf['hashtable']['params']['compression'] = false;
$conf['hashtable']['params']['large_items'] = true;
$conf['hashtable']['driver'] = 'Memcache';

As soon as I enable memcache this way, I see some "Invalid  
application" errors in Horde and I get randomly "User is not  
authorized for horde" messages in the Horde log, followed by logout.  
No other error messages. I set memcached logging to -vv and see no  
activity at all.

I suppose I do something wrong in the Horde config but after a lot of  
reading and searching I got a bit lost now. Any pointers in the right  
direction welcome!

Running latest Horde Groupware 5.1.4 on Ubuntu Server 12.04