Upgraded the OpenVPN daemon on one side of the connection to v2.0, and now it can't connect.

Starting with version 2.0-beta17, OpenVPN's default port number has been changed from 5000 to 1194 per an official port assignment by IANA in November 2004. If you are mixing different OpenVPN versions on both sides of the connection, make sure to explicitly use a port directive, and not rely on the default value.