I found this weird behavior while importing video files from my camera. These files have a # sign in their file name (Clip #127.mov). If I open them with the read node I cannot read them, and the viewer displays an error with the text "cannot load frame". Manually removing the # from the filename solves it.
Oddly enough however, the button "File info..." in the read node does display meaningful results.
Natron version 2.3.4 on Mac OS X High Sierra