Unfortunately we were not able to reproduce the issue.
plugin_dir_path() doesn't seem to be loaded on your site, or at least not before any of the plugins are. That could be caused by an issue with your WordPress installation, or with your server configuration.
Could you try to reinstall WordPress?
If that doesn't help, could you check your site with the Sucuri SiteCheck tool to make it hasn't been corrupted?
Let me know how it goes.