I had parentheses in a filename and they broke the decryption command. I think that will happen with other characters that have a function in the shell. The app run ok but just outputted the errors in the decryption area:
sh: -c: line 0: syntax error near unexpected token `('
sh: -c: line 0: `/usr/local/bin/pass filename-with-(parentheses)'
I guess just quoting the files where you run the external commands would do the job but I'm no CPP expert... I think it's here: https://github.com/IJHack/qtpass/blob/master/mainwindow.cpp#L189