Welcome to the forum @Nicholas_Marlow
I’m not finding that exact message. Might it have looked like below?
If so, the JavaScript in the web browser (which is what) said that, and you can use its tools to look at this.
Most browsers have some sort of web developer tool available via F12 or a menu to look at network work.
Is Duplicati run as a systemd or other service, or run manually? Does ps show it stay steady or restart?
Do they show up in the Restore dropdown? That’s late in the backup itself, but other steps run even later.
Have you watched the status bar to see what it says before disconnect occurs? Do you send any email?
Mail sending failing after upgrade to Debian 11 might be relevant, but details are thin, both here and there.
Does the backup job run on a schedule? If a scheduled backup doesn’t run, it will run on Duplicati restart.
With GUI coming and going, you can do better with log-file=<path> log-file-log-level=information (at least).