I installed Qt5.15 from the maintenance tool and tested new version of Qt Multimedia on Qt example named "Media Player Example". 

1) In Qt.5.15 :

If I open a video located in "/Users/chloe/Downloads/sample.mp4" it works but if I open a video located in "/Users/chloe/Downloads/sam ple.mp4", the media player seems to stays in "Loading" status. 

I tested with others videos containing spaces in their filename, media player always stays in "Loading" status. 

2) In Qt 5.14.2 : It worked

If I open a video located in "/Users/chloe/Downloads/sample.mp4" it works and if I open a video located in "/Users/chloe/Downloads/sam ple.mp4", it works too.

3) Try to use Video QML type in a simple project Qt5.15 : 

I encountered same problem : if an url with spaces is set to source attribute, my Video Item status attribute stays in "MediaPlayer.Loading" value. There is no error detected in error or errorString attributes of my Video Item. So, this bug does not seem to concern only the example "Media Player Example".

Regards