We keep seeing this exceptions throughout the day on the main TeamCity page on TeamCity 8.0.4 - this used to be not a problem we ever saw on TC 7.
We use an external SQL database housed on a fairly high powered DB server - this should not be the issue.
Today this for the first time failed a build when a step refused to start.
Unexpected exception SQLServerException: SQL error when doing: Fetching query results SQL query: select bs.id, bs.branch_name from build_state bs join history h on bs.id = h.build_state_id where bs.modification_id <= ? and bs.build_id < ? and bs.build_type_id = ? and bs.id <> ? and bs.is_changes_detached = 0 and bs.is_deleted = 0 and ( h.status <= ? and h.status <> 0) order by bs.modification_id desc, bs.build_id desc SQL exception: The query processor could not start the necessary thread resources for parallel query execution.