Blank screen with Fatal error, when system plugin is enabled

Those issues both seem to indicate that the main /components/com_jfbconnect files were not installed correctly. Can you re-install JFBConnect so that the files are (hopefully) installed properly? That should fix things, but if not, let us know.

One think you can look for is if the following file exists:
/components/com_jfbconnect/libraries/factory.php
If that's missing, it means that something is preventing the files from being copied to the correct directories on installation, and you'll need to narrow down what's going wrong. It's likely a permissions or ownership issues on those directories.

Thanks,
Alex