I downloaded the app and ran the installer on my desktop, then again in mr, I've uninstalled and reinstalled, but I can only open on the desktop, where it just shows black with a white dot in the middle.
sorovlorov MR as in Mixed Reality? Have you restarted computer? Do you have all the necessary codecs perhaps? We recommend:
HEVC Video extensions (https://www.microsoft.com/en-us/p/hevc-video-extensions-from-device-manufacturer/9n4wgh0z6vhq)
And one of those two:
K-Lite codec pack (https://www.codecguide.com/download_kl.htm)
LAV filters (https://github.com/Nevcairiel/LAVFilters/releases)