After buying another TL-R600VPNv2 and restoring the settings from the first one we still had the problem. Apparently something was corrupted in the configuration file. I reset the router to factory...

Model : Hardware Version : Firmware Version : ISP : On our network with about 80 users our TL-R600VPNv2 will block a random user from the internet but they will have local resources with no problem....