There is a "log" directory in your Ubooquity working directory.

You can also use the "view logs" links in the General settings section (although you will only see the latest lines).

To use this feature, "Folder metadata display" has to be enabled in the advanced options and comics should be grouped by folder (not "flat").

Apart from the name ("folder.jpg" or png or gif), there is no restriction.

My mistake, I'll put it online again and update the tutorial link.

Any error or warning in the logs ?

In the comics and books configuration section, for each directory you share, you have to specify which user is authorized to access it.

If I understand correctly what you did, you created the user but did not authorize any directory for him to see. Hence the empty page.