[horde] Login timing out early

[Chris St. Pierre]

A number of my users have reported that they are being logged off
apparently at random.  Looking at the logs, I see the follow error
correlated with the "random" logoffs:

[client 67.48.82.85] PHP Warning:  ldap_mod_replace(): Modify: Insufficient access in /var/www/html/horde-3.1.3/lib/Horde/Prefs/ldap.php on line 501, referer: https://ellen.nebrwesleyan.edu/horde/imp/mailbox.php?mailbox=INBOX
[client 67.48.82.85] PHP Warning:  ldap_mod_replace(): Modify: Insufficient access in /var/www/html/horde-3.1.3/lib/Horde/Prefs/ldap.php on line 501, referer: https://ellen.nebrwesleyan.edu/horde/services/portal/sidebar.php

We are using LDAP logins and preference.  I've looked in the LDAP logs
and sure enough, I'm getting err=49, LDAP_INVALID_CREDENTIALS.  What
could be going on here?  Is Horde using a single persistent LDAP
connection that could be timing out?  Or is it trying to reconnect on
each page and, for whatever reason, forgetting the user's password?

Thanks for your help!

Chris St. Pierre
Unix Systems Administrator
Nebraska Wesleyan University