If i trust google translate, the message says: "could not receive data from client: Connection timed out" which clearly indicates a database problem (as the 900+ secs times).
Do you have MaxIdle and/or MaxOpen set for pool1 or pool2? If so, remove your settings.
Concerning your questions: ns_cache is completely agnostic about the database. The worker threads (e.g. connection threads) require db-handles (references to db-connections) via gethandle, they require in most cases a pool1 connection and sometimes a pool2 connection (for subqueries). The handles are released either explicitly (happens seldomly) or automatically when a request is done and the worker thread is done.
