EV 8.0.4 Offline Vault Issue - how to troubleshoot?

Hi,

Users having offline vault issue

Some users getting error  – Office 2010 | “synchronisation has failed”

Some users getting error – Office 2007 | “there are no vaults available”

EV Server 8.0.4.1991

Client: 9.0.1.1038 - HTTP-only Outlook Add-In version 9 installed with Outlook 2007 or 2010

Note: Both EV server and client server has proxy configured for internet connection.

What troubleshooting steps we can take to resolve this issue pelase?

Many thanks in advance