For those of you using WordPress and Jetpack, here is an important update for you. It seems that version 5.1 of the Jetpack plugin requires you to be using PHP v7.0.
I recently upgraded a client's Jetpack plugin to v5.1 (the latest version at the time of writing this article) and subsequently found the site returning a 500 error. I realised that the only change I had made to the site was to upgrade Jetpack, so the first thing I did was deactivate that plugin. That fixed the problem.
500 Internal Server Error is a generic error message, given when no more specific message is suitable. There are a number of causes for a 500 Internal Server Error to display in a web browser.
So I looked at the Jetpack forums and found that the latest code for the plugin has dependencies on PHP v7.0. There are quite a few new features in PHP v7.0 (see a full list here), so some of these changes were not compatible with the version of PHP currently being used for my customer's wordpress site.
WordPress' official support documentation also suggests that the latest version of the platform is optimised to run under PHP v7.0, so I upgraded the underlying PHP version, re-activated the Jetpack plugin and the issue was fixed.
So, if you have upgraded Jetpack to v5.1, make sure you troubleshoot the site and be sure that it's working properly. If you have access to your hosting configuration, changing the PHP version is quite simple. However, if someone else manages the underlying codebase, you will need to contact them BEFORE upgrading Jetpack.
Jetpack is a great plugin, so don't let this small issue stop you using it.