This change affects only users which have the "general.see_all" permission, normally Administrators and Guests. But the most affected users will be the admins.
Previous Check_MK versions had the options "Visibility of Hosts/Services" and "Visibility of Hosts/Services (Webservice)" which could be set in the user profile. These options could be used by all users which have the "see all" permission to limit the shown hosts and services to only the ones which they are really a contact for.
The later option has been deprecated now. It was first integrated to make users of external tools like Nagstamon, which fetches it's list of host and service problems from the Check_MK webservice, be able to show only the the problems of their hosts and services. But this option also had a side effect: When it is enabled and the option "Visibility of Hosts/Services" was not enabled, the admin could see all hosts/services in the GUI but when executing a CSV export he was only getting the hosts/services he is a contact for.
To fix this situation we decided to deprecate the "Visibility of Hosts/Services (Webservice)" and replace it with a URL variable which can be used to filter each page individually. The new URL variable force_authuser can be used on any view and set to:
The now deprecated option will still be working as before but you are recommended to uncheck the option and use the new mechanism. If you are using it for Nagstamon you will need to use a Nagstamon 2.0 release newer than ALPHA-20160307 once it is available.