Plex Web App - will not automatically plex next video in playlist




I'm currently running PMS 3.20.8 on Windows 10 x64
Recently i have noticed an issue when playing a playlist or shuffle on the Web app, it will not automatically play the next video.
This is happening with all video containers - MP4, AVI and MKV
When the 1st video finishes the display is dimmed and the orange spinning circle is displayed, but it wont load the next video unless i manually click the "next" button.
I did work on a previous version of PMS ( about a month or so ago)

I have tried playing a playlist shuffle on different supported browsers / devices but just end up with the spinning circle. I have also tried from the desktop the PMS is running on, still with the same issue.

I have also recently rebuilt the desktop with Windows 10 and reinstalled PMS, rebuilt the database with all default settings - but still the issue persists on all devices using the Web app player.

The only device I have that will automatically play the next video in a playlist is an old Roku Stick.

Hope someone can help me with this