Thank you very much for your help.
I´ve set a "Reap Interval" in "Manager Properties" and "Store Properties". Additionally I´ve set a "Session Timeout" in "Session Properties".
I have restarted the server and tested several days, but the problem still remains. The number of active sessions grows and grows - with the same speed like before!
I really don´t have a clue what is going wrong here ...
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