how to trouble shoot - RuntimeError: Open Failed

[stas poritskiy]

hey guys,
I am working on application. App is processing a list of files in folders.
Folders act as a NEW-LOOP. so if all files in one folder had been worked on, file is then saved and next folder is picked up.
it works fine only if i have a SINGLE folder, however, when another folder is there, i get this RuntimeError: Open Failed.
I checked if those could be the files by leaving only single (another folder), however all was fine.
Questions - How can i trouble shoot what is causing the problem? i don't know where to begin.

Thanks!