When I try to sign any PDF by using File ��� Digital Signatures ��� Sign Existing
PDF..., Draw opens properly and I see the signing bar, but clicking "Sign
Document" results in an "Error" window saying that
"/path/to/the/directory/where/the/pdf/is/located/4l0ngr4nd0mstr1ng.tmp does not
exist." This window pops up twice and the dialog for signing the PDF never
appears.

If I do create that file (by just doing a touch), the button doesn't do
anything (in the latest version of LibreOffice, in the version shipped with
15.1 it says that there is a signature and it's in an old format).

Using the signing functionality of exporting the file as a PDF works fine. It
also works fine to sign Writer documents.

I tried with the version of LibreOffice that is default in Leap 15.1, as well
as the latest version from OBS, same issue in both cases.