Thanks for checking it our Polina,
I tried the latest beta today on the the new BigSur Beta 4 and still had the same problem.
I first removed the old version, then installed the latest version of Viewer.
Unfortunately, the application starts and then hangs before displaying the connection window.
The tool bar is visable on the MAC OS toolbar, however when you hover the mouse over any of the menus the mouse turns to a spinning wait icon.
The only way to exit the app is to force close it.
If I am able to supply any diagnosits to you please feel free to ask.
Cheers