Any ideas?
The mailbox servers need to be the same version. You need to upgrade
the Node 1 to Sp2.
Follow the steps in the link depending on what type of CMS you have.
http://technet.microsoft.com/en-us/library/bb676559(EXCHG.80).aspx.
Make sure that both nodes are at the same level of updates and
software. Make sure that you don't have different administrator
passwords. Try keeping both the same in all areas.
Hope this helps you!