GnuPG key import fails

I have freshly installed Debian 10 Buster (stable) and wanted to import the backup I made with old stable (9, Stretch). Seahorse 3.30.1.1 shows me the backup file but nothing happens when I select it and click on "import".

However, the import works instantly when I use the command line. (Afterwards I can use Seahorse to delete keys for example.)

I use an old Acer Extensa 5220 with 2 GB of RAM.