Try Home → About → Show log → Stored, and click on any entry from about that time.
<backup> → Show log tends not to get fatal backup errors, but you could look there too.
If you are possibly having upload problems, you could start with a smaller backup initially.
Having the initial backup done will let you resume future interrupted backups more easily.
You can also look at that job log’s Complete log → BackendStatistics → RetryAttempts.
Some retries are normal, but a lot might indicate that you are having some transfer issue.
Using what Storage Type on the Destination screen?
What sort of test? Test connection is very basic. Duplicati.CommandLine.BackendTester.exe is better.
Neither of those stresses a connection as much as a big backup. Any idea how much data went up OK?