Yes agree to Ruby 2.7. And don’t use plain global jekyll which is 4.
Within your project only use bundle exec jekyll which 3.9 and that is the appropriate version for GH Pages.
If you still have issues, check your config and frontmatter and any references to paths in your files. It looks like an unexpected data type on your code. But then again it might not be your code and it is a system issue.
If you can move all your content of your repo temporarily to another directory so you only have Gemfile, vendor and an almost empty index.md (not even config), then you can run the serve command there and then you’ll see that Jekyll can work alone. Then start bringing your code back in to the directory and see which file causes a break if at all.
You can also setup a new directory locally to test Jekyll itself using a template
cd my-repo
bundle exec jekyll new ~/repos/test
cd !$ # Same as cd ~/repos/test
# maybe bundle install steps.
# ...
bundle exec jekyll serve --trace
Please provide a repo link too if you figure out that you can get Jekyll to run for a new project but not your existing one.