By the way, since you're calling LB::server pool from HTTP_REQUEST, it should be checking against your default pool.
It looks like this is the codeshare you're using:
http://devcentral.f5.com/wiki/default.aspx/iRules/Cookie_Encryption_across_pools_and_services.html
What's your default pool called? Your TCL error leads me to believe the rule is somehow looking for a pool called "cookie"
What version are you running?
The reason you aren't seeing anything for logging LB::server is because you haven't chosen a server which makes me think your default pool isn't specified.