This error is usually caused by incorrect server configuration. It typically means that a security tool like ModSecurity, Bad Behaviour or Suhosin is blocking or filtering the menu data.
For example, I've received similar reports from another user where it turned out the "save changes" request was modified by an excessively paranoid firewall that thought it was an SQL injection attempt because the menu data contained keywords like "update" and "set". Of course, in practice the request they were completely benign and the keywords were part of settings like "update_plugins" (a built-in capability string) and "Settings" (a menu title).
I would recommend asking your hosting provider to check their security-related logs for recently blocked/filtered POST requests. They should be able to add an exception for this plugin.