Hi,
from what I gathered from the other thread, you are running the standard OpenVPN client on Windows. Unfortunately, I do not have experience with that particular OpenVPN client software when it comes to OTP - on Linux clients, however, OpenVPN behaves properly when reauthenticating and asking for a new OTP.
Since IPFire’s OpenVPN server does not difference between client operating systems in that regard, I assume the problem is related to your OpenVPN client, rather than IPFire. 
Thanks, and best regards,
Peter Müller