PGP signature isn't selected when trying to sign during export of PDF-file in LibreOffice Writer

I have asked the following question on ask.libreoffice.org but nobody answered:

I have an Enigmail PGP signature that I use in Thunderbird under Ubuntu Linux. In Libre Office I can use it to sign (and encrypt) an ODT-document. But when I try to use it during export of a document as a PDF document something goes wrong. In the 'PDF Options' window on the 'Digital Signature' tab, I click on the 'Select' button and get the 'Select Certificate' window. My signature is already selected (it's the only one present). I now click on 'Sign'. The window closes, but the signature isn't shown in the field called "Use this certificate to digitally sign PDF documents:". So when I press the 'Export' button, I get a "signature generation failed" error message, and export of the PDF file fails.

Can anyone help me please?

I also tried signing an existing PDF-file, following the instructions here. In other words:

1. Open LibreOffice Draw
2. File-menu -> Digital Signatures -> Sign existing PDF.
3. Select/open document.
4. Press 'Sign Document' button.

But here, my signature isn't shown, so I can't select it and sign with it.