I think I run into the pinentry problem discussed here: https://www.enigmail.net/support/gnupg2_issues.php .
Here is my log:
2015-03-31 08:30:14.501 [DEBUG] enigmailCommon.jsm: parseErrorOutput(): return with errorMsg = gpg-agent[49083]: command get_passphrase failed: Operation cancelled
gpg: cancelled by user
gpg: skipped "0x16FB185B": Operation cancelled
gpg: [stdin]: clearsign failed: Operation cancelled
So I should change to a different version of pinentry. However, there is no alternative:
[wolff@UltraLight:~]ls -la /usr/local/Cellar/pinentry/0.9.0/bin/pinentry*
lrwxr-xr-x 1 wolff admin 15 9 Nov 05:12 /usr/local/Cellar/pinentry/0.9.0/bin/pinentry -> pinentry-curses
-r-xr-xr-x 1 wolff admin 52688 9 Nov 05:12 /usr/local/Cellar/pinentry/0.9.0/bin/pinentry-curses
As you can see there is just pintentry-curses. I use the most recent version of pinentry for Homebrew:
[wolff@UltraLight:~]brew upgrade pinentry
Error: pinentry 0.9.0 already installed
I suspect that no installation of Enigmail with Homebrew's GNU PG 2 on Mac OS X works.
As you're using a GUI system, you need a GUI version of pinentry.
You need to install pinentry-mac. See e.g. https://github.com/Homebrew/homebrew/blob/master/Library/Formula/pinentry-mac.rb
Works now - thanks a lot! Maybe you want to add a note to https://www.enigmail.net/support/gnupg2_issues.php ?