I have 2910 with 2 ADSL 8mb through 2 vigor 100 modem. When a pc is logged into something that requires password (MSN Messenger, Internet Banking etc.) a WAN IP change for a new IP session causes a log-out (or failed log-in). The only way to circumvent this seems to be to bind a particular IP to one WAN port - which defeats the object of the load balancing. Is there solution to this problem?

set up a load balance rule for TCP port 443 so that only wan1 for instance will use it (with auto failover enabled for the rule). That should stop secure sites from breaking.
Port 1863 might work for MSN messenger.