No certificates show up when trying to sign a document

I’m using LibreOffice 6.0.7.3 from the Ubuntu 18.04 repository. I’m trying to sign a document with my GPG key using this menu:

If I click the “Start Certificate Manager” button, it opens seahorse and I can see my GPG key:

However, if I click “Sign Document”, the key does not show up:

How do I sign a document? What am I doing wrong? Does it have something to do with the “Certificates” section of seahorse?

What do the details of the key tell? Certificates are issued for various functions sign, certificate and encrypt. Thus check you key is issued for signing and has a expiration date in the future Right click -> Properties -> Tab: Details in seahorse on the key).

There are two entries in subkeys, one of them says “Sign, Certify”, the other says “Encrypt”. The “Sign, Certify” key has the ID I recognize and says it expires “Never”. The type is “RSA”, strength is “2048”, status is “Good”.

Did you get a solution to this? I’m having the exact same problem. I need to digitally sign a document for university, but libreoffice isn’t displaying any of my certificates that I create to sign the document.

Hi, I have exactly the same problem. How to get someone’s attention to look at this problem ?