I changed nothing except updating the Nuxt dependency from beta 6 to beta 8.
I suspect the problem might be obvious to someone who is familiar with the internals of Nuxt and what changed from beta 6 to beta 8 based on this error message. I had a look through the commits from 6 to 8 and nothing jumped out at me.
If it's not apparent what the problem is I'm more than happy to help narrow down the cause, but it would probably take a while.