There is a Fedora bug report 
https://bugzilla.redhat.com/show_bug.cgi?id=812421 reporting that openvpn 
is having problems because of the line
SUBSYSTEM=="net", KERNEL=="tap*", ACTION=="add", RUN+="/etc/xen/scripts/vif-setup $env{ACTION} type_if=tap"
in /etc/udev/rules.d/xen-backend.rules which is causing the xen script to 
run when openvpn tries to use a tap device, causing it to fail. I have 
used the attached patch to solve this problem, by matching the form of the 
tap device that xen uses more exactly to avoid to openvpn case. A better 
long-term solution (suggested in one of the comments in the bug) might be 
to use a more specific name instead of "tap" so we have less chance of 
interfering with another application.

 	Michael Young