I'd assume using the Windows 10 Insider Preview builds would be considered a unsupported configuration by JRiver.
Likely reason why this happens is because with each Insider Preview updates, Windows does an in-place upgrade with the existing/previous install. Because of that and the changes to the system with each update, the part(s) MC relies on for handling licensing (e.g. hostname, among other things) likely changes with each update which in turn causes MC to lose its registration. I believe the same thing might happen to when installing the latest Windows 10 update every 6 or so months.
I'd personally advise against running MC on Insider Previews builds. Any of those builds could break MC or one of the components it uses due to the volatile and buggy nature of the Insider Previews. That, and I don't think JRiver's devs would want to download and mess/waste their time supporting each new Insider Preview build. Better to wait for each RTM update and support that once released.
That's my two cents and opinion, at least.