This is a self-answered question in case anyone else is looking. The Summer '19 sandboxes dropped over the weekend and I started getting the above error this morning. It was only on 1 Aura component but it was a base component so affected lots else. The component refused to save and ceased displaying in the community.
Turns out the component had a css file that only contained '/**/'. I deleted the file and all is working again.
Removing empty CSS comments (/**/) from the CSS file will solve the issue. Also, I've noticed that components are not getting loaded in community builder for the same reason. Removing empty CSS comments from CSS files will fix all these issues.