Thank you muppeth for the prompt reply. I've been looking at many different options to answer your questions.
I used username and password to authenticate my access to the Disroot cloud. I did find that the error disappeared when I used the URL of the actual keepass2 file. Something like:
When I tried to synchronise using the full URL, username and password, keepass2 said that it was synchronised but the entry added to Disroot via the Android keepass2 app didn't appear in the database on the Ubuntu keepass2 app even though I sychronised to the Disroot file from the Android app.
Unless you have seen this behaviour before it seems like an error with the Ubuntu keepass2 app not synchronising properly with the Disroot database.
I need to investigate further.