OABGen encountered an error while generating the changes.oab file for version 2 and 3 differential downloads of address list '\Global Address List'. The offline address book has not been updated so clients will not be able to download the current set of changes. Check other logged events to find the cause of this error.
On the Exchange server, start Regedit.exe.
Open the following registry key:
Right-click Parameters, point to New, and then click DWORD Value.
For the new value name, type OAL post full if diff fails.
Double-click OAL post full if diff fails. In Value data, type 1. Click OK.
Microsoft Exchange > Organisation Configuration > Mailbox > Offline Address Book > Right click on Default Offline Address List > Update
Then in Outlook,download address book.