SharePoint Search Server keeps throwing errors

In the Central Admin. there are no errors reported.

Upon the user attempting to perform a search they get the message:

Sorry, something went wrong. Search has encountered a problem that prevents results from being returned. If the issue persists, please contact your administrator.

In the event log there are the following issues (in the application log):

Event 2548 (SharePoint Server Search) of severity 'Error' occurred 550 more time(s) and was suppressed in the event log

In the ULS log the following two errors keep occurring:

Content Plugin can not be initialized - list of CSS addresses is not set.

and

Failed to extract required parameter FastConnector:ContentDistributor, hr=0x80070002 [pluginconfig.cpp:81] search\native\gather\plugins\contentpi\pluginconfig.cpp