Where are you looking? Duplicati currently (I believe) has warning popups that send one to the job log at Job → Show log → General, however the server component of Duplicati (used if you use the browser GUI) can leave messages at About → Show log → Stored. If somehow that’s not enough, one can try watching About → Show log → Live, or setting up a –log-file. If you can find a warning, please detail it in a new support post.