an update: after re-activating the Jevelin theme after trying the 2017 theme, the 404 error seems to have gone away. Weird.
I did, it was the demo content for Portfolio.
I activated the 2017 theme and now I can view my portfolio pages, so it must be something with the Jevelin theme. I will make a new thread so I don’t take over yours @chr1s9r
Following along because I am having the same issue.