Hi,
Reviewing the latest log we can see the following. So the error has changed slightly now since the change of webdav protocol.
6/8/2010 1:39:30 PM [2420,61] [WebDAVRequest::Send] Exception sending WebDAV request: System.Net.WebException: The remote server returned an error: (403) Forbidden.
I know you mentioned that SLL is enabled to the ISA Server however this is not where SSL settings etc end with Exchange 2007 generally. You need to confirm if SSL is enabled on the mailbox server as per below. Depending on if SSL is enabled or not HTTP or HTTPS will be the correct settings for the webdavprotocol line I gave you earlier.
2. Open IIS and browse to the Exchange VD.
3. Right-click on the Exchange VD => Properties => Directory Security Tab
4. Select "Secure communications" => Edit. Determine if "Require secure channel (SSL)" is active.
5. Ensure the entry below is configured in the Web.config file on the CAS to reflect the requirement on the Exchange VD (HTTP/HTTPS).
Though Gertjan makes a valid point is not going to assist in this particular case. The ExchangeServers.txt file resides on the Enterprise Vault Servers and assists with the config of the EVANON virtual directory. This will be called when you attempt to open archive items for example. If this comes from a NLB address and this is not allowed for the EVANON directory then it would need to be added to the ExchangeServers.txt file. It does not assist with the rendering of the EV buttons. In this case the Webdav call that is failing is to the mailbox server to read the hidden message in the user's mailbox. When it cannot read this hidden message which at present it is failing for it will not be able to show any EV buttons. This is your problem and why the buttons are not being shown.