it's the same for me and it only happens with Qlikview.
So it's probably a database issue but also maybe a database issue coming from QVConnect.
We have discovered that when our server has not enough memory (less than 4GB out of 64GB), it may crash with database errors like Connection Failed probably because of QV Connect. The database is fine and everything's running correctly every where except with Qlikview.
Difficult to make a case because it's a random error. Sometimes it works sometimes it doesn't ...