Dear Mantainer, I've discovered that I was suffering from two different problems, and I've found workarounds for both of them.
The problem with greyed out folders occurred because the subwindow with the list of files opens filtering out all non .nbk files. For some reason, folders without the extension .nbk are greyed out instead of being shown normally, but if the filter is reset to all files, *and the folder reloaded*, then they appear normally and notebooks can be loaded normally too. I still think greying out non .nbk folders is a bug, but at least it's an easy one to circumvent. The reason I was not able to load a notebook from the command line interface (or other interfaces) was that I did the tests with a notebook I copied from another machine that was using a different character set and, for some reason, keepnote was unable to load files with non standard characters (accents and such) in their names that were using that character set. I could solve the problem just by renaming the files removing non standard characters.
