The usual cause for this (from what I have seen) is when someone tried to take the same cert+key used to connect to an OpenVPN server, and copy that same cert+key to another machine and then use the same cert+key to connect with both clients to the same server at the same time. The default behaviour for an OpenVPN server is to not allow the same cert+key to be used to connect at the same time. Whichever one connects last "gets" the vpn link and the old one loses out. Though the "tun" interface may remain "up" on the previous client to use it, it is dead.
If you own the OpenVPN server, you can do the (usually wrong) thing and change this setting:
duplicate-cn
Check the man page for details.
That is usually the wrong choice for many reasons. Please do not choose that.
If you are trying to re-use the same cert+key, the "right" answer is to make a new cert+key/config for the new client, and make a new one for each and every new client that you want to connect concurrently. Then, if there is an incident where a key is lost, falls into the hands of your "enemy" or more likely is copied/stolen, you can "revoke" the compromised key, update your CRL on your server, and issue a new one, blocking anyone from using the revoked key(s).
Posting logging details, and samples of your configs with any "secret" or "identifiable" information *removed*, using the "[" + "code" + "]" BB code can often help people act on actual output from an app instead of blindly guessing and making assumptions like me. In this case, logging details from the *server* side around the time the new client connects would probably be the most helpful, but also include logs from each client around the time of the switch just in case they have data to show a cause for this behaviour.
