Plex requests proxy was removed during testing due to bugs (such as what you are experiencing) and it should not be until in the current stable branch. Are you using the latest version?
Edit: current open issue with github. There is some consideration about moving back to the meteor version.
Please do remember that we do not maintain these projects and while sometimes we introduce bugs, more often than not the bug lies within the project itself. It is prudent to open issues (or at least search) with the developers of such projects