This error is related to caching and compression. Wish I had time to dig deeper now, but I don't.

For us, it happened when a database was moved to a new server without turning off anonymous page caching first.

Solved by:

1. Adding the following line to settings.php:

$conf['page_compression'] = FALSE

2. Logging into the site using Firefox (not Safari) and going to http://www.example.com/admin/config/development/performance and hitting the Clear All Caches button.

It's not enough to just truncate the cache tables in your database.

3. Go back and undo your settings.php change.

Topic: Drupal