Users use an SSO solution with Confluence (for instance a Kerberos SSO)
The following error is displayed:
Please also check this article to see if the result source size isn't the issue: Search Result do not work sometimes
You will need to whitelist the OpenSearch URLin the SSO configuration.
Please whitelist the following URL: http://<confluence-baseurl>/plugins/servlet/opensearch/1.0 or use https when you have an SSL certificate enabled.
You can also specify an IP from which the whitelist is only active to make sure that only the SharePoint Search Service is able to use the whitelist.