Same happens in 3.0 and 3.1 
What could it be?
Works with today’s Blender(3.2)-Build.
First off, welcome to the community, @Alexey_Ost! I have a theory that might explain things. I see you are using 3.2.0 Alpha. There’s a chance that something is broken in that build that causes your .Blend file to not work with the Viewer node/Backdrop in the compositor. That file may carry the problem to other builds - 3.0 or 3.1. Does this happen with all files in all versions, or just this file? If not, I recommend making a new file in whichever version has a working Viewer node, and append your assets from your broken file to the working file, and save a new version.
If that’s not the case, let us know and we’ll try to think of something else!
Thanks! Just… I’ve been in this place from the very beginning… I lost my password a long time ago, I’m — O.din13 
Does this happen with all files in all versions, or just this file?
Yes, this happens in all files and versions greater than 3.0, as well as with default settings. The last time I saw it — 3.0
Ho-ho! In new daily build all works again! Great! :: )