I searched in forums and it seems to always be a problem with the preview browser. There were a few fixes to 'Project model unavailable' assert, ashley fixed it in backend C2 though rather than a specific fix to someone's project. Could be an actual bug which is why I say to post it there. If it's a huge project or something and you don't want it shared maybe you can email it straight to scirra, not sure what happens in that situation.