For uninteresting reasons, I’m not permitted to use either iCloud or Dropbox on my primary Agenda machine.
I now have a new computer to replace my old one, and I need to move all my Agenda data from the old computer to the new one.
I’ve tried copying the com.momenta.agenda.macos folder from Containers onto the new computer, but Agenda is still completely empty on the new computer.
Is there a step I’m missing?
You can simply export all your Agenda data from your old computer as an Agenda file and import it on your new Mac.
You can simply export all your Agenda data from your old computer as an Agenda file and import it on your new Mac.
I would not recommend that as it would loose your categories and project structure. Normally you’d use indeed Dropbox or iCloud sync to reinstate your library on the new machine, but since that’s not an option you would need to not only copy the com.momenta.agenda.macos folder from ~/Library/Containers, but also the WRBK2Z2EG7.group.com.momenta.agenda.macos folder from ~/Library/Group Containers. Do make sure Agenda is not running when you copy/place these files. Hope that helps.
Okay, that’s interesting to know.
Whereby now the question arises for me on which occasions one should use the import/export function. 
I have learned that it is better not to use it for transferring complete agenda files. Should you use this function only for single projects or whole categories – or better only for single notes?
As long as you stick with one agenda file per project it works fine, or if you want to share notes with someone else, but when moving many projects to a new Mac you want to use the above instructions.