When the currently opened archive is either empty (or doesn't contain any image files, for that matter), the "Automatically open next archive" functionality no longer works. Attached patch should fix the problem.
Fix opening next/previous archive when current archive is empty
Neglected that this bug not only affects opening the next archive, but also going back one archive. Replaced the patch with a somewhat fixed version.
Thanks. Applied into my fork repository (github.com/HoverHell/Comix/).