[horde] Blank page after login

[Tom Conway]

I get this error now:

An error has occurred. Furthermore, Horde encountered an error attempting
to log this error. Please check your Horde logging configuration in
horde/config/horde.php.
[line 191 of /internet/horde/lib/Horde.php]

Hitting the BACK browser button gets me to the inbox

in /horde/config/horde.php my loggin is setup ...

$conf['log']['type'] = 'file';
$conf['log']['name'] = '/tmp/horde.log';

What am I missing?