If you click on the VPN button you will notice that it lists how much data you have sent through the VPN for the month.
I feel confident asserting that once you pass a certain amount of traffic, your VPN access will go "unavailable" until a certain cooldown passes.
I have two machines running Opera in front of me right now. The VPN service is functioning on one, but not the other. Same configuration, same IP... but different amounts of VPN data usage and thus (I believe) different rights to VPN access.
I have no problem with a cooldown system on a free service, but I do think it ought to be made more clear what is going on rather than giving a message that implies a technical glitch.
Thats make sense, Im with you, they really need to do some notification about the "VPN" thing to the community.