The NTLM connection is limited by Qlik to 5 sessions. Sometimes, while testing the connection or doing multiple analysis, the session count is exceeded. Then you have to wait for Qlik to timeout the sessions, usually about 15 minutes.
If you are getting intermittent connection errors using NTLM, you can resolve by using a CERTIFICATE or QLIKLOGIN connection instead.
If this does not describe or resolve your problem, please post the error you receive when you try to analyze an app.