I assume from the error message that gpg-agent tries to launch pinentry, and this fails. The usual reason for the failure is that "pinentry" points to a command line version of pinentry and not a graphical one like pinentry-gtk or pinentry-qt (which is required if you use Enigmail).
enigmail> /usr/local/bin/gpg --charset utf-8 --display-charset utf-8 --batch --no-tty --status-fd 2 --comment Using GnuPG with undefined - http://www.enigmail.net/ -t --clearsign -u 0x3D0D6F68 --use-agent
gpg: problem with the agent: End of file
gpg: skipped "0x3D0D6F68": Operation cancelled
gpg: [stdin]: clearsign failed: Operation cancelled
enigmail> /usr/local/bin/gpg --charset utf-8 --display-charset utf-8 --batch --no-tty --status-fd 2 -a --export 0x3D0D6F68
I assume from the error message that gpg-agent tries to launch pinentry, and this fails. The usual reason for the failure is that "pinentry" points to a command line version of pinentry and not a graphical one like pinentry-gtk or pinentry-qt (which is required if you use Enigmail).
See also this article: http://www.enigmail.net/forum/viewtopic.php?f=10&t=249