Does that mean you ssh in, and your browser access is remote? If so, maybe you hit this upgrade issue:
Duplicati upgrade - “The host header sent by the client is not allowed”
I’m not sure how far it gets if one hits it though. Above description of failure doesn’t detail symptoms either.
–webservice-allowed-hostnames: The hostnames that are accepted, separated with semicolons. If any of the hostnames are “*”, all hostnames are allowed and the hostname checking is disabled.
as seen on server help, using a “*”, might be a test. I think Settings menu might have a nicer edit interface. Alternatively the issue referenced at the top of this post suggests ssh port forwarding instead of remote on whatever network you have. This is also more secure than opening Duplicati on less-than-trusted network.