Just had this problem and fixed as follows.
It was not just the jetpack.php that had the problem- also hooks.php and extras.php in the same folder.
When I viewed the server version of the files the code was condensed, on the back up file it was not- simply copied the back up content and pasted it over the server file, saved and problem fixed.
I then had to go to /wp-login.php instead of my usual /wp-admin.php route to the dashboard.
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This reply was modified 8 years ago by russellh1.
