So maybe this will save someone else some headaches...
I was trying to configure OpenVPN Server on my Buffalo router, running the standard Professional firmware (DD-WRT variant) from April 2011, and it just would not run. The /var/log/openvpn log file would gripe about "Options error: Unrecognized option or missing parameter(s) in /tmp/openvpncl/openvpn.conf:XX: mssfixfragment (2.1.3)"
The GUI in that version is a bit flawed in this area, and it will generate an option in /tmp/openvpn/openvpn.conf named "mssfixfragment". it will cause openvpn fail to start.
So that is the wrong text, it should be mssfix and then fragment. I found the following bug ticket:
"Sash" marks it as fixed without any explanation. So I pulled down the BrainSlayer 16994 build and loaded it (webupgrade version), and all is well now and the config file is created correctly.