From a newsgroup post:
Error: Item has already been added. Key in dictionary: "<email address 1>, <email address 2>" Key being added: "<email address 1>, <email address 2>"".
I finally got this to work after I realized that you can specify only one mailbox per start-to-end configuration tag. For example:
<Mailbox EMailAddress=""<email address 1>">
Originally I was grouping more than one mailbox in the individual XML configuration mailbox group settings. This caused the changes in the Global Settings to be ignored and revert back to the defaults. I also noticed this event in the log.
It seems that Event 4098 can mean many things and even the description text may change. In my case, it was having a problem finding a path as part of the TraceListenerFile attribute. It comes out that the path it was trying to reference did not exist. I created it and the error disappeared.
This behavior manifests when Resource Mailboxes are no longer being managed by Auto Accept Agent on Exchange Server 2003. You can fix this problem by unbinding each Resource Mailbox using RegisterMailbox.vbs with the /U switch and then reregister each Resource Mailbox using the same command without the /U switch.