I use okular for annotating some of my PDFs but have been having an issue recently where I can only open files that are on my SSD where I installed ubuntu, but not on the 2nd hard drive (located at /mnt/...).
When run from terminal the output I get is:
Qt: Session management error: None of the authentication protocols specified are supported org.kde.knotifications: Audio notification requested, but sound file from notifyrc file was not found, aborting audio notification
When doing the same to the file on the SSD I get the first message (though not the second, but I think that is just it wanting to use sound to warn me that it couldn't open the file). I have also run it under sudo just incase it was a permission error and get
[error] cannot open locale definition file `en': No such file or directory No protocol specified qt.qpa.xcb: could not connect to display :1 qt.qpa.plugin: Could not load the Qt platform plugin "xcb" in "" even though it was found. This application failed to start because no Qt platform plugin could be initialized. Reinstalling the application may fix this problem.
I have also tried with other programs and don't see this distinction (including being able to open the same file on the 2nd HDD in a different program).