[horde] Class definition of Horde_Cache_emcache not found

[d.hill at yournetplus.com]

I'm running Horde Groupware Webmail 1.2.3 on FreeBSD 6.4. Periodically  
I am receiving the error in the subject line of this message. It  
happens at different times, I.e. moving messages to a folder, viewing  
a message, viewing the message headers, etc.

The error message displayed is:

A fatal error has occurred
Class definition of Horde_Cache_emcache not found.
[line 107 of /usr/local/www/apache22/data/horde/imp/lib/IMAP/MessageCache.php]

Does anyone know what possibly may be going on?