QlikView Sessions are not released (Session and Usage CALs consumed) as expected and hang around until Service restart.
Session are started and documents opened according to the Event log, but no sessions are being closed in the Session Log. Eventually the QlikView Server Service is restarted to clear them out.
A single user or multiple ones are consuming all the sessions. The session log does not record most of them, or shows empty Client Build versions.
The QlikView Server log constantly logs...
Information Session stop reason: Socket closed by client
QlikView 12.20 (November 2017) and up
End Users with Internet Explorer Plugin 11.20 and earlier
Unaligned Internet Explorer Plugin versions.
End Users may still be using old, outdated Internet Explorer Plugin versions, such as 11.20 and earlier.
This can be verified by looking at the installations of the particular end users, or by reviewing the session logs. See Session log: Client Build Version is empty in QlikView November 2017
on how to identify them.
While not all 11.20 plugins will be affected, updating all users is highly recommended. See: Qlik View and IE-plugin is not backwards compatible between major releases
For information on how to best proceed with the upgrade, review: Enterprise Solution for QlikView IE-plugin/Server Version Mismatch