If you get "a problem has occurred with repllog.exe". when trying to sync a Windows Mobile 6.1 phone with exchange server the actual underling error is hidden by Windows Error reporting. Once this is disabled the actual error is 0x8503001c
This seems to be caused by a network error when trying to do the inital download over a 3g connection via the internet. The solution is to sync directly with the Exchange server using a local network either with a VPN connection or using an internal wifi solution.

1) Setup the Outlook account on the phone to sync with the internal name of the server eg EXCHANGENAME
2) Sync with the server
3) When the sync is complete change the server name to the external name of the server eg WEBNAME.COM
4) Sync away

This fixed a problem I had with a brand new Treo Pro and Exchange Server 2003