QlikView Server (QVS) stops showing graphical elements in AJAX client

After a few days of usage, users stop seeing charts and other graphical elements in dashboards. Only a spinning wheel is shown, until the services are restarted. All kind of charts are affected, tables are not. Any user can be affected, and any app as well. Once the issue starts happening, it can some users might be able to see the charts in some apps, but then issue will eventually occur. Sometimes, closing and reopening the session fixes the problem temporarly, but the only fix that works for a few days is restarting the services. Then the issue comes back. It looks like the servers might reach a threshold of maximum number of graphical objects that can be shown.
The Desktop Client and the IE plugin are not affected.

Environment:

QlikView 11.20, any
QlikView 12.20, any

Upgrade to QlikView 12.30 (November 2018) or higher: the new rendering engine based on SKIA will solve this, not having the same limitations of the old GDI+ engine.
Should an upgrade not be possible in the short time, follow How to clear the cache used by QlikView Server and QlikSense Engine to have a regular cache cleanup.