In SharePoint 2010, 6644 refers to error related to Event Receiver. In MOSS 2007, is this the same? MSDN doesn't specified what type of error this event id refers to.
In the event log, I got the following, multiple times
Event Type: Error Event Source: Windows SharePoint Services 3 Event Category: General Event ID: 6644 Date: 12/7/2012 Time: 4:10:58 AM User: N/A Computer: xxxxxxxxxxxx Description: Event manager error: Could not load file or assembly 'xxxxxxxx.xxxxxxxxxxxxx, Version=18.104.22.168, Culture=neutral, PublicKeyToken=123456789abcdef' or one of its dependencies. The system cannot find the file specified. For more information, see Help and Support Center at http://go.microsoft.com/fwlink/events.asp.