I'm not sure why that one doesn't include the error code.
https://github.com/koreader/koreader.../main.lua#L591
You could try quickly replacing that line with this so we know what the server's saying:
[code] text = "Response code: "..code, })
Given your other message, it'll probably be something like 401 unauthorized though.
Could you double check the wallabag settings in a text editor on your PC? That's in /mnt/onboard/.adds/koreader/settings/wallabag.lua.