Signing failed -- "No secret key", even though I have the key

[azarus]

Hello GPG users,

I have a problem regarding signing data. Whenever I try clear-signing,
this appears:

gpg: writing to stdout
-----BEGIN PGP SIGNED MESSAGE-----
Hash: SHA256

hello
gpg: signing failed: No secret key
gpg: [stdin]: clear-sign failed: No secret key

I invoked clearsign like this:

echo "hello" | gpg --sign-with <my-key-id> --clearsign

This is what gpg -K lists:

/home/azarus/.gnupg/pubring.kbx
-------------------------------
sec   rsa4096 2016-12-20 [SC]
      <my-key-id>
uid           [ultimate] <my-user-name> <my-email>
uid           [ultimate] <my-real-name> <my-other-email>
ssb   rsa4096 2016-12-20 [E]
ssb#  rsa4096 2017-06-23 [SE]

Can somebody explain what I'm doing wrong? This was working a couple of
days ago, I even reset my .gnupg directory from a backup, with  no
success.

Thanks for the help!

All the best,

azarus