stuck 'locking existing frames' forever

I'm having a strange issue where Previews get stuck 'locking existing frames'.

I'm happily working, previewing different comps and then all of a sudden the info window will stop displaying the preview rendering info when I hit numpad 0, the green bar in the timeline time ruler shows that frames are being rendered but nothing in the info window. Hit numpad 0 again and the info window says 'locking existing frames' forever !

The only way to fix the issue is to restart the program.

I'm running the latest version of Ae, Windows 10, 64GB of RAM, GTX 1060, and M.2 for boot and source drives.

Any suggestions greatly appreciated...