Debugger not working on Hosted workbench on spfx webpart

I have spent a number of months working on a custom SPFX WebPart using standard tools and React. As of Wednesday last week I have been unable to successfully use gulp serve with Chrome to hit any break points to investigate why our latest dev build is failing to fire a third party call.

I am using SharePoint Framework 1.7.1 and chrome 64 bit v 74

I see no indicative errors or warnings in console that tell me anything of note but my colleague is getting an error stating 'Failed to load debug manifests' 

Can anyone suggest a solution