So yeah, once you have set it up on your desktop, you should not set it up on your mobile device but add the mnemonic from your desktop there instead - the mnenomic is how the different clients are able to see the files between them…
So step 6, that it asks for your android e2e mnemonic/passphrase but doesn’t accept the client one, that should work. I mean, really - that is how it should work…
The other way around should also work, you create it first in Ubuntu (after wiping all data on server and clients!) and then you have to enter the mnemonic on the desktop.
To clean the keys properly, see Help test the latest version of E2EE! -> you have to clean keys on the server but also on the (desktop) client.