I think that the svg files shouldn't be in the 22x22 directory. Maybe that causes problems, which would explain why changing the Icon key in the desktop file to "seahorse.png" fixes the issue.
On the other hand, I tried deleting the svg icons and copying the files from /usr/share/pixmaps/seahorse/22x22/ to /use/share/icons/hicolor/22x22/apps/, but this didn't help. So the problem might not be related to the svg files.
I think that the svg files shouldn't be in the 22x22 directory. Maybe that causes problems, which would explain why changing the Icon key in the desktop file to "seahorse.png" fixes the issue.
On the other hand, I tried deleting the svg icons and copying the files from /usr/share/ pixmaps/ seahorse/ 22x22/ to /use/share/ icons/hicolor/ 22x22/apps/ , but this didn't help. So the problem might not be related to the svg files.