From my host.
the 500 was due to your account hitting process/memory limits. That’s something that should never happen in a normal situation.
But what looked to be happening was that processes for your site were doing something very CPU intensive, and never finishing before additional processes started up. They started slowing down because they’re all competing for the same CPU, and the processes just started to stack up. Eventually this resulted in failures.
Rolling back the plugin immediately fixed that – requests for pages on your site come in, a PHP process starts up, and finishes the request in a fraction of a second.
You’re welcome to send this info to the dev – if they have any questions/something to try, please copy in the exact text of their message and we’ll take a look