BIZARRE! I didn't know C1v5 had this issue.
Does anyone know what one might be able to do to manually give it a kick to start rendering the files? I would hate to restart and take chunks of time to load the session.
Sorry to hear you're having these issues (my Win7 setup has been running stable since the first few updates), but (therefore) it seems something hardware related might have caused it. Maybe deleting the previews folder content, can help? I'm not 100% sure, because I've turned to working in Catalogs, but I believe that Session previews/thumbnails are stored in folders called "\CaptureOne\Cache\Proxies\*.cop" within the relevant image folders. You can then also try to change the preview image size, which could perhaps somehow affect the preview building time, but I suppose it could also add time to render previews at a larger size when you zoom in to process specific files.
Most issues seem to be caused by hardware related things like graphics card drivers. You may want to temporarily switch GPU acceleration off to see if it prevents these issues from happening again, if you get things running again.