[horde] ActiveSync stops working after migration from mod_php to mod_proxy_fcgi and PHP-FPM

[Arjen de Korte]

I have migrated a (Apache 2.4) server from mod_php to mod_proxy_fcgi  
and PHP-FPM. This almost works, the only thing is that ActiveSync  
seems to have stopped working. In the webserver logs I see lots of  
lines like

     192.168.1.133 - - [11/Jan/2014:12:39:33 +0100] "POST  
/Microsoft-Server-ActiveSync?Cmd=Sync&User=arjen&DeviceId=847CC7DCA0F0B1CF&DeviceType=Android HTTP/1.1" 401  
-

After enabling ActiveSync debug logging at the same time the following  
is logged

     2014-01-11T12:39:33+01:00 INFO: [12589]  
Horde_Core_ActiveSync_Driver::authenticate() attempt for arjen
     2014-01-11T12:39:33+01:00 NOTICE: Device failed to pass the user password.

The same installation is operational again if I disable mod_proxy_fcgi  
and enable mod_php5, without further changes. What am I missing here?