Soon after the release of Jetpack 3.0, users reported problems with the spell check module which utilizes the After The Deadline service. In Jetpack 3.0, when users proofread a post, they are greeted with a General 500 error. The error also shows up if you’ve configured it to automatically proofread a post before publishing.
The Jetpack team is aware of the issue and is working on a point release that will be available soon. Until then, if you want to apply the fix before the release, navigate to jetpack / modules / after-the-deadline / proxy.php and replace the Proxy.php file with this one. The error has to do with AtD calling an undefined function in proxy.php. A detailed explanation can be viewed via the following ticket on Github.
The Jetpack development team has done a fantastic job fixing problems discovered soon after a release. Earlier this year, the team rapidly put together an update shortly after the release of 2.9 to fix a bug with the spell checking module that prevented posts from being published. Only eight days separated the release of 2.9 from 2.9.1.
Jetpack 3.0.1 is available for download. You should see an upgrade notice soon in your dashboard. This release fixes the spelling and grammar module and contains several other bug fixes.