If I had to guess, you have different versions of Jekyll (and friends) running locally vs. on the server. The fact that you’re getting a deprecation warning about
plugins is your first clue.
If you’re not using a
Gemfile locally and on the server you might want to. That will keep your gems in check and help enforce that you’re using the same versions in both environments.
The only difference will be prepending Jekyll commands with
bundle exec… eg.
bundle exec jekyll serve