In retrospect, I think my errors were different than what @wkaa saw, but the core problem was the same. The /tmp wasn’t present.
I’m painfully new at Docker, so I had to tiptoe along with Claude to find the issue based on the interpretive clue @kolaente provided.
At first I had a secondary problem because when I opened the console for the vikunja Docker container (on my unraid server), it briefly flashed and closed. Claude helped me establish that this was, in fact, expected as I’m running the Vikunja container I found in the Unraid app store which is a distroless container image.
Hence no console and no /tmp.
Once I created that directory from the Unraid console and mapped it in the Docker container settings, the export process proceeded correctly.
Well, mostly. The export initiated correctly but said it would notify me by email, which I don’t have set up. After waiting a bit I was able to download the export even without being notified, so that’s good.
But perhaps a valuable update to the export flow would be to have some form of notification show up in Vikunja itself when the export completes.
Similarly, the original failed export was not surfaced in the UI. There was no sign of an issue so it wasn’t clear that I should go log diving to find the error.