Note that this problem can also be caused by two clients sharing a single license. Unlike Named CALs that can be leased to a maximum of two platforms at the same time (effectively doubling your license), a QlikView license can only be used in a single AccessPoint session at a time.
Imagine that you log into two Windows machines using the same account, and both visit a document in the Access Point at the same time. Whenever one session makes a selection in this document, it will reconnect to the document and throw out the other session that is making use of the same license. The second session - upon accessing the document - will get a Connection Lost message and try to reconnect, thereby closing the first session.
This does not apply when opening two sessions in a single browser or on the same machine.