On server: GZip and WP Super Cache
This happens when apache2 on the server is configured to GZip at page load time. This results in double-compression. Disabling WP Super Cache compression fixes the issue. Just let GZip compress the pages.
Uncheck this: Compress pages so they’re served more quickly to visitors. (Recommended)
You can leave it checked if you are using a 3rd party plugin for Gzip comperssion like GZip Ninja Speed Compression. Just uninstall the GZip plugin and let WP Super Cache do the compression.
You can also add this to your htaccess file to prevent this from happening again.
AddCharset utf-8 .html
AddCharset utf-8 .json
AddEncoding gzip .gz