OK - did a bit more testing to help you recreate the problem. Seems to only occur when using SSL with IMAP.
I have to admit that it seems quite an inconsistent problem, while testing I have successfully sent and received numerous encrypted emails to myself, however Thunderbird then suddenly started crashing the moment it connected to the IMAP server.
At times, I have also received error code 12263 from my IMAP server while using SSL.
Setup information:
Vista Home Premium SP1 32 bit
Thunderbird 2.0.0.21
Secure Email 2.0.0.3
IMAP with SSL port: 993 <-- added to IMAP protocol settings in CSE.
SMTP port: 25
I am now doing all testing with server timeout at 90 seconds and auto-decrypt off.