openvpn stops working a short period after connecting

Oh number one cause of this in our company is the same OpenVPN key being used simultaneously in two different places. The same IP gets allocated by the server to both clients, causing a dropout after a minute or two.

That smells like a client-side routing problem. Take a look at the routing table immediately after the OpenVPN tunnel establishes (while it's working), and then again after it stops working, and look for any differences. I've had a problem where a DHCP client was wiping out (overriding) routes from OpenVPN, which would make the connection to the OpenVPN server unavailable.