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I use version 2.1 in the free version on a Qnap TS251+ NAS and maybe I will upgrade to the Pro-Version. As far as I understand, a purchased license can be used for all further 2.x- Versions, but not for 3.* and higher. So first I want to ask, when a "3"- Version will be launched or how long will the "2"- Version will be actual?

I moved your question to the appropriate location.

Version 1.0 was released in July 2012, and 2.0 was released in May 2019. So it was 7 years between major versions. So I'd guess that we still have from 5-7 years before the next major version appears.