pan: error sending posts

In Debian bug 945315, a user reports:

pan stopped sending posts.

After "Send Article" it says:

There were problems with the post. / Error: bad email address. / Go back Continue Anyway

After "Continue anyway" task is hanging.

In pan's Events register I can see

Posting of "Pst sub" failed: 441 Duplicate Content-Type: header

I've reproduced this problem with Debian's pan 0.146 (which is compiled with Gtk3)

All the best

Dod