Update at 6.22pm BST
Jetpack have confirmed the problem: “If your site uses PHP 5.2 or PHP 5.3 and if you previously used Jetpack’s Sitemaps feature, your site will break.” To be fair to Jetpack, those are pretty old versions of PHP. Time to move some of my sites over to a server that supports a more up-to-date version of PHP, I guess. Better information and further action than what I originally published is in their update post: Fatal Error after updating to the most recent version of Jetpack? Read this.
Original post published at 8.24am BST
A few WordPress sites I manage went down overnight and the culprit is the Jetpack plugin. Jetpack auto-updated itself to version 4.8 which then took the sites down, resulting in the ‘white screen of death’.
I strongly suspect that there’s something in the Jetpack update that requires php 7, as the sites I have running on php 7.1 are doing fine – it’s the sites running on php 5.3 that went down.
The (temporary) fix
It’s a backend thing, I’m afraid. I had to go to my site’s files via FTP and rename the folder /wp-content/plugins/jetpack to jetpackxxx. It doesn’t really matter what you rename it to, just make sure it’s a name that isn’t already used. Once you’ve done that, you should be able to get to your site’s WP admin screen. Head to the plugins screen and you should see a message saying jetpack’s been deactivated because of a problem with the folder. That is A Good Thing.
Optional step: if you have a backup – and you really should – you could upload the older Jetpack files into the plugins folder. This basically rolls you back to version 4.71, assuming you have a recent backup. I left Jetpack deactivated after doing this to prevent it auto-updating to the site-killer version but will manually update once I’m happy they’ve patched whatever it is in version 4.8 that’s killed some sites.