Is your user flagging archived or non-archived mails for follow up when this occurs?
Could you please send me over some log files from his machine? They will be here:
%userprofile%\\Local Settings\\Application Data\\Red Gate\\Logs\\ESA Outlook Add-In 1
If you could also zip the following directory on the machine with the services installed that would also be useful:
%allusersprofile%\\Application Data\\Red Gate\\Logs
Please e-mail those to email@example.com
quoting reference F0026996.
Also, is this Outlook 2003 or 2007?