Was this issue ever resolved?
I too have seen this error, but only on one user with IE8. I watched the error happen and it was having problems loading emails and handling mouse clicks. I almost wonder if it is something with the javascript rendering on the local machine.
Culley
Mail | Dell 2950III | 2x Quad Core 5420 | 8gb RAM | 6x 146gb SAS RAID 0+1 | Red Hat 5.3 | Zimbra 6.0.10 Network Edition
Test | VMware ESXi Whitebox | Phenom II Black 3.2ghz | 12gb RAM | 6x 1tb SATA RAID 0+1 | CentOS 5.4 | FOSS, Not in use now