Version:           1.6.82 (using KDE 3.2.91 (CVS >= 20040706), compiled sources)
Compiler:          gcc version 3.2.2 (Mandrake Linux 9.1 3.2.2-3mdk)
OS:                Linux (i686) release 2.4.25

I get the following message(in a popup-dialog) when trying to sign a message in KMail:
"The signing operation failed for an unknown reason."
I have gnupg version:
$ gpg --version
gpg (GnuPG) 1.2.2

It works fine in Thunderbird(with enigmail).

In .xsession-errors I see:
kmail: CharFreq returned 3/0.98066 and I chose quoted-printable
kmail: Converting LF to CRLF (see RfC 2633, 3.1.1 Canonicalization)
Format info for openpgp/mime:
  Signing keys: 387681F4
  SplitInfo #0 encryption keys: 387681F4 387681F4
  SplitInfo #0 recipients: andreak@officenet.no
GpgME::Data(): DataProvider supports: read, write, seek, release
GpgME::Data(): DataProvider supports: read, write, seek, release
QGpgME::QByteArrayDataProvider::read( 0x8961088, 4096 )
QGpgME::QByteArrayDataProvider::read( 0x8961088, 4096 )
ASSERT: "!mSignature.isNull()" in messagecomposer.cpp (1881)

From messagecomposer.cpp:1881:
  Q_ASSERT( !mSignature.isNull() ); // if you hit this, check gpg-agent is running, then blame gpgme.
I don't have any program named gpg-agent, and it is definitely not running while working in Thunderbird.
Any hints?