As suggested in post #15, I just filled a bug report on GitLab: <https://gitlab.gnome.org/GNOME/seahorse/issues/232>, however, as I have my doubts if it is really an upstream bug, as it didn't occur on Ubuntu 16.04, just on Ubuntu 18.04, and some users reported that in Debian it works.
As suggested in post #15, I just filled a bug report on GitLab: <https:/ /gitlab. gnome.org/ GNOME/seahorse/ issues/ 232>, however, as I have my doubts if it is really an upstream bug, as it didn't occur on Ubuntu 16.04, just on Ubuntu 18.04, and some users reported that in Debian it works.
Hope anyone will be able (and willing) to fix it.