[horde] 10-webmail.php

[Gary]

I've had a recurring issue involving the file  
/imp/conf/prefs.d/10-webmail.php. If I delete this file imp works  
fine, but with this file present I get this error immediately when the  
login screen appears and when ever I attempt to write a new message:

2013-06-11T03:10:29+00:00 WARN: HORDE [imp] PHP ERROR:  
include(/var/www/horde/lib/../imp/config/prefs.d/10-webmail.php):  
failed to open stream: Permission denied [pid 6956 on line 355 of  
"/usr/share/php/Horde.php"]

2013-06-11T03:10:29+00:00 WARN: HORDE [imp] PHP ERROR: include():  
Failed opening  
'/var/www/horde/lib/../imp/config/prefs.d/10-webmail.php' for  
inclusion (include_path='/var/www/horde/lib:.:/usr/share/php') [pid  
6956 on line 355 of "/usr/share/php/Horde.php"]

Having this file present also causes imp preferences gui to fail,  
stating that there are no preferences for this application. The file  
has 664 permissions with www-data (apache user) as the owner. Although  
this is not a show stopper, I sometimes wonder if it's not a symptom  
of some other configuration issue.

Has anyone had this problem and found the reason?

TIA
Gary