I wanted to follow up on this thread for the benefit of all you readers from the future!
I was unable to get 1.3x to function on the new Soekris 6501 and had to use 1.81 on those units. I'm currently, and successfully, running in a mixed m0n0wall environment with two of my four locations running 1.34a (on old Soekris 4801 units) and communicating with m0n0wall 1.81 via IPsec with no errors for over a month now. I also run many mobile VPN clients off of my main unit and they aren't reporting any issues either.
Hope this helps someone.
Edited to fix - it was a 6501 not 6801. The 6801 isn't due out until Q1 2015.