I wondered if that might be the case, I suspect it is that the new certificates are no longer compatible with the early versions of TB! I honestly cannot think of any way to resolve this one, other than updating to a later version of The Bat! sorry. :'(
Having said that, I resisted upgrading from v3.6 until I went from WinXP to Win7, when I had to upgrade as Win7 would not even install v3.6, so I am now on the latest v6.7.7.0 (2014 Xmas Edition) and it is pretty good. Better than v3.6 which I was very happy with, and for the cost of the upgrade I figured it wasn't worth the hassle of using a different email program and having to do things differently.