You may see this message when trying to use OpenVPN to connect into a draytek router and the user on that router that you're using to authenticate has a static IP defined. This is an intermittent issue and is related to how the draytek router manages static IP's assigned to VPN's.

The draytek router will log something similar;

Basically, there's no fix for this, when an OpenVPN session disconnected, the router doesn't release the IP address right away so when trying to reconnect it cannot then allocate it to the new session. Rebooting the router of course fixes this, but it'll happen again. The only option is to give it a short whilst between disconnect and reconnect and it will be ok.