I've got exactly the same problem. I'm using mbox instead of imaps, so that
isn't the source of the problem. There's a method_missing in the trace.

--- TypeError from thread: main
can't convert nil into String
/var/lib/gems/1.9.1/gems/sup-0.10.2/lib/sup/mbox/loader.rb:117:in `exists?'
/var/lib/gems/1.9.1/gems/sup-0.10.2/lib/sup/mbox/loader.rb:117:in `store_message'
/var/lib/gems/1.9.1/gems/sup-0.10.2/lib/sup/util.rb:599:in `__pass'
/var/lib/gems/1.9.1/gems/sup-0.10.2/lib/sup/util.rb:586:in `method_missing'
/var/lib/gems/1.9.1/gems/sup-0.10.2/lib/sup/sent.rb:28:in `write_sent_message'
/var/lib/gems/1.9.1/gems/sup-0.10.2/lib/sup/util.rb:559:in `method_missing'
/var/lib/gems/1.9.1/gems/sup-0.10.2/lib/sup/modes/edit-message-mode.rb:325:in
`send_message'
/var/lib/gems/1.9.1/gems/sup-0.10.2/lib/sup/mode.rb:51:in `handle_input'
/var/lib/gems/1.9.1/gems/sup-0.10.2/lib/sup/buffer.rb:270:in `handle_input'
/var/lib/gems/1.9.1/gems/sup-0.10.2/bin/sup:270:in `<module:Redwood>'
/var/lib/gems/1.9.1/gems/sup-0.10.2/bin/sup:64:in `<top (required)>'
/var/lib/gems/1.9.1/bin/sup:19:in `load'
/var/lib/gems/1.9.1/bin/sup:19:in `<main>'

Quite often (~50% of trials) an email gets sent but right after that sup tries to 
save it in the INBOX.Sent folder and crashes. Sometimes it works though. Here's 
the stripped config for that folder:

sources.yaml:
- !masanjin.net,2006-10-01/Redwood/IMAP
  uri: imaps://mail.example.com/INBOX.Sent
  ...
  usual: false
  archived: false
  id: 2
  labels: []

config.yaml:
:sent_source: imaps://mail.idiles.com/INBOX.Sent

Thank you!