Recently we came across an issue with our SharePoint 2010 people search. It did not work. The All Site scope worked fine but the people search scope did not return any results. We were using SharePoint 2010 FAST search.
The people scope with FAST search was in FAST Query SSA (How does People Search work using Fast?). We needed to look at the FAST search configuration in Central Admin: Manage Service Applications >> Service Applications
We made sure that the Content Sources of the FAST Query SSA had the Start Address set to: sps3://<server name>
We run a full crawl on this content source. At the end of the crawling process the Crawl Log had one Error: Access denied ..
The Crawler could not access the User Profile. To solve this we needed to allow the crawler access to the User Profile Application. This was done through - Manage Service Applications >> Service Applications >> User Profile Application. Then add the permission “Retrieve People Data for Search Crawlers” to the account which run the FAST Query SSA.
Finally we re-run full crawl on the FAST Query SSA.