You are most likely using an old version of mInstaller (one that is no longer compatible with our servers).

Please close the app (either via CMD+Q or using the Activity Monitor), delete from your Applications folder (be sure to also delete all other existing copies), empty your trash(!), and download the latest version:
​https://www.motionvfx.com/page/minstaller.html

Do not launch it yet though and instead, go to your Applications folder, press and hold the CMD+Option(Alt) keys, double-click mInstaller.app, and when asked to, delete the app's preferences.
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Finally, try logging in to mInstaller and the problem should be solved.