I can't reproduce it!!
did it bug again (with the same error) when you had that output that you just pasted ?
It works fine here.. and by the way, the bug is from base64, I thought that maybe amsn was giving it something wrong, but it doesn't look like it, I tried manually :
% ::MSNOIM::parseFieldEncoding "=?utf-8?B?=wqDCoFVyaWVs?= "
  Uriel
I tried with :
% package require base64
2.3.2
and with :
% package require base64
2.2.2
press Ctrl-Shift-C to open the console and type the "package require base64" to see which version you have... maybe the version you have is older/newer and contains this bug...
Try also "package versions" to see the versions you have installed on your system :
% package versions base64
2.3.2 2.2.2
if you have 2.3.2 or 2.2.2, then grep for 'base64' and 'package' and where we do the "package require base64" change it into "package require -exact base64 2.3.2" .. see if that helps...