Weird, really weird. No idea where the /unknown comes from. Now it works:
# apt install grommunio-admin-api-mod-vts
Installing:
grommunio-admin-api-mod-vts
Summary:
Upgrading: 0, Installing: 1, Removing: 0, Not Upgrading: 0
Download size: 0 B / 15.3 kB
Space needed: 25.6 kB / 1,061 MB available
(Reading database ... 72843 files and directories currently installed.)
Preparing to unpack .../grommunio-admin-api-mod-vts_1.20.43.mbb64403-1+18.1_all.deb ...
Unpacking grommunio-admin-api-mod-vts (1.20.43.mbb64403-1+18.1) ...
Setting up grommunio-admin-api-mod-vts (1.20.43.mbb64403-1+18.1) ...
gr:~# apt install grommunio-admin-api-mod-vts
grommunio-admin-api-mod-vts is already the newest version (1.20.43.mbb64403-1+18.1).
Summary:
Upgrading: 0, Installing: 0, Removing: 0, Not Upgrading: 0