Hi,
If the issue came from out of the blue, that may indicate a problem on the server side. Error 500 means, that the server does not respond to the request.
These issues are extremely rare and mostly caused by server related problems – or if the plugin was downgraded from a newer release.
For starters, I strongly recommend updating the plugin to the latest possible release – from what I can see on the screen, that is an very outdated version, which could explain the problem as well.
While we are trying to maintain the updates as much as we can, it is possible, that the update may fail because of the very old version. Try updating, then clearing every layers of cache. I will attach the latest beta version to this reply, please us that for the update.
If there are issues after the update, then go for a full plugin wipe and then install again, that will resolve that.
Do not ever downgrade though – that will definitely cause a lot of issues, as the plugin data will remain updated in the database, and cause a lot of issues.