[L4D2] Session is no longer available problem

[Dtor]

I'm having this problem too , in my problem when my server loaded finale map and start executing finale script it will .... well activate total server lockdown . Player will play through finale map and move to the next campaign then you can join the server normal .
If server reach this condition it will realease the lock.
#1 : All players must finish the finale map and change new campaign
#2 : All players died and still free slot in the server
#3 : All players left the server and it will execute reset campaign.