FortiClient - Received non-IPv4 compliant network address / Create VPN network interface failed

I got the following error when I tried to connect to a VPN using FortiClient:
Create VPN network interface failed

/var/log/forticlient/sslvpn.log gave some more details:
vif:78 Received non-IPv4 compliant network address
vpn_connection:1231 Create VPN network interface failed

Together with a colleague we found out that the cause was that too many slots on the VPN was taken. If the colleague disconnected then I could connect and vice versa.

So the error means that all VPN login slots are used.