I experienced this error with some frequency with the 'Encrypt draft messages on saving' option enabled. It can be quite annoying. I first thought it was related to my OpenPGP smartcard. But after doing some research I figured it out it must be because I blew away my trust database. The problem is that the error is confusing and uninformative for users; it doesn't supply any meaningful information. This has been previously filed, so I was curious why it was marked 'fixed in 1.8.0' when it's still around.
Bug #306 (https://sourceforge.net/p/enigmail/bugs/306/)
We thought we fixed it. Can you please attach a debug log file (using menu Enigmail > Debugging Options > View Log)?
I had similiar error messages with 1.8.1
Could you please attach a debug log file? Otherwise there is nothing for us to analyze. It requires about 1 minute to do that.
Here's a log.
Maybe connected (because I'm experiencing it in the same context), maybe an independent problem: Changing the identitiy during mail-creation isn't regognized by Enigmail until the editor part of the composing windows receives the focus again (judged by the informations in the "Enigmail status bar"). So writing your mail, changing the identity afterwards and then directly clicking on "Send" oder "Save" leads to misbehaviour.
All known occurrences of this bug were fixed with v1.8.2