Issue:
The customer is having issues with OTP not working from stand-alone relay servers, such as in the DMZ.
Solution:
Check the server time on stand-alone relay. It's possible the time is out of sync from the internal server. OTP relies on the servers being within 20 seconds of each other.
The best way to do so, is to set all the servers to use the same reliable NTP source.
The NTP Pool Project is a very reliable NTP source: