You must dive inside the playground file ("show contents") and rename "section-1.swift" to "contents.swift".

Hi, same problem here. However, I seem to have found a temporary solution. My way is to keep clicking on “retry” until the system eventually starts up (it may take between 5 and 15 minutes usually). I know it is exasperating, but it is the only workaround I’ve managed to find. I’ve tried any other method, such as reinstalling OS from the system itself and from Recovery partition, used Disk Utility to check disks, volumes and partitions, use third part apps to check volume structure and perform checks) and nothing seems to work. I remember having this very same problem with the same laptop but once it eventually stopped happening (maybe after a major software update, but cannot tell for sure).