I have just checked through the perl code for ovpnmain.cgi which covers the OpenVPN pages including the Advanced Options section.
The Keep Alive values are set at 10 and 60 by default and included into the server.conf file by default. They will only change if they are edited by the user. The code checks that only numbers have been used and that the second number is greater than the first. If that check fails then an error message is placed at the top of the page.
I have checked this back to 2011 and the code for that item has not been changed since.
I have checked my production IPFire and several of my vm ipfires and all of them have the line
keepalive 10 60
in them.
I think if your server.conf ended up with that line missing some problem must have occurred but if you shut down the OpenVPN server, go to the Advanced Options page, confirm that the values are 10 and 60, or whatever you want them to be and press the save button then the server.conf file will be updated with the entries (you can check to confirm) and you can re-start the OpenVPN server.