I have a 1:1 weight ratio between my wan links and there is a problem with ZS forum that doesn’t support cookies. I never had a problem with https sites however.
Not sure about how most BB software packages are built but I do know that I have programmed a couple of sites that checks the IP address of the web browser against that in my session information. The idea was to keep from having someone spoof the user’s login by replaying the cookie. (I had to relax this because places like AOL have multiple proxy gateways and the end user’s request may come from a group of IP addresses. However all the providers I saw hitting my web sites had their proxies in the same /24 subnet.)
If you are logging into a site programmed like that and each page load comes from a different IP address then it would kick you out.