gzdecode relates to gzip functionality inside Apache and/or Wordpress. follow the gzdecode error, go upstream, you may have to perform a re-install of libraries.
I would look at your php_info() output and see what libraries are active in php and modules on in apache.
I would look in those places with your server configuration. php and apache may need to be restarted once you've reinstalled the compression libs.
Either way, I would roll-up a new instance of your site from backups from 30 days prior, then apply all the updates for WordPress, Apache, Linux, etc (not sure where this site sits for you and how much access/control you have.) And then start pushing data refreshes in to bring you forward to the last week of your data.
Pull down all relevant log files from that server.
Install a WordPress Security plugin. There's lots of good ones out there. Be Proactive and keep up with your updates and backups.
Best of luck.