Yeah, sounds like you have the same problem that a lot of people are having.
OpenVPN chokes when the bandwidth of its WAN is saturated.
You see that error, and then OpenVPN restarts itself in 10-30 seconds (or so).
These other threads all have the same problem from the looks of it, but no bug has been filed yet.
https://forum.pfsense.org/index.php?topic=75989.0
https://forum.pfsense.org/index.php?topic=76735.0
https://forum.pfsense.org/index.php?topic=77169.0
I'm hoping this gets filed soon. The problem has been present in 2.1.1, 2.1.2, and 2.1.3 (64bit) (versions I've tried)
-nb