issue connecting with exchange server

hi all,

we got a small networking infrstructure running sbs 2003 R2 and some PCs
running vista /outlook 2003 . these PCs have exchange configured in their
outlook. from time when they click on send\receive , they get the following
error :

task 'Microsoft Exchange server' reported error (0x80040115)

no such error has been noted with PCs running XP/office 2003. After
searching the internet , ive come across this article:

http://support.microsoft.com/kb/939321

this solution works for one day...the next day the error reappears.

Also note that ISA 2004 that comes with SBS is also installed in the
environment.

Please advise,

Kryshzt