Hello, I have a big problem 
Audacity says that it missing 444 audio data block files. Data seems not corrupted, because there are files in there and they can be played. When checking log, I see that Audacity tries to open files, which aren’t in data subfolders. If I select "treat missing audio as silence (this session only), Audacity finds 252 files, which are unused in this project. After that it opens two tracks, which was muted when I saved project, all unmuted tracks now are silent… I did nothing, nothing was deleted or corrupted by me, but seems that Audacity changed files names by itself and now can not find these files… 
Please help me, because I worked many hours on that project. Also, I copied original .aup and data.
seems that Audacity changed files names by itself
That’s a clue. What did you call the projects? The exact filenames?
You should only use lower case letters, upper case letters, numbers, -dash- and underscore in filenames. Other characters can carry meaning you didn’t intend. Slash marks can mean a change of folder or directory and your project can get lost.
Dates can cause problems. Use USO dates in filenames. Today is 2020-05-03, or 20200503. Not 5/03/20.
Koz
This is a Project.
The AUP file is a text file with the instructions for Audacity to put all the files in the _DATA folder together into your show. You should be able to double click the AUP file and the show will open.
The AUP file and the _DATA folder need to be in the same location or folder for the show to open.
You can open an AUP file in a text editor and read it. Don’t save anything. It contains instructions with semi-English words like this.
Note the original project name is burned into the AUP file. That’s why you can’t change a Project name later-outside of Audacity.
Did you save any copies of the show outside of the Audacity Project?
Koz