Hi,
Please use plain text instead of formatted one in your email messages: it is a annoying...
> I did what you suggested me: I downloaded psi-plus-portable-1.3.398_win7_x86_64.7z and it worked. I was delighted to see Psi work again, then I closed it.
> When I tried to open it again with psi-plus-portable.exe or psi-plus-webkit-portable.exe, it didn't work! Windows 10 doesn't recognize it.
What do you mean when you said that you have closed a program? By default Psi IM and Psi+ are configured to stay in a system tray when user closes their windows. You have to click on "Exit" menu item explicitly to finish the application completely. In case of doubts launch Task Manager and look for psi-plus process there...
> I had to install Psi again from psi-plus-portable-1.3.398_win7_x86_64.7z and it worked.
You are doing something wrong...
> I changed psi-plus-portable.exe to psi-plus.exe, it opened, but each time I open Psi this way, I have to configurate it again!
1) You may move the folder with Psi+ configs from folder with portable version of Psi+ to default user path, for example to: C:\Users\<your-username>\Local Settings\Application Data\Psi+\
2) You may set an environment variable PSIDATADIR to any path in your system. (But have in mind that your user should have write access to this folder.)
> What I noticed when I wanted to uninstall it is that it doesn't appear in the list of programmes to be deleted, neither in Windows, nor Revo Uninstaller, nor CCleaner. So it is not considered as a programme?
It absolutely normal situation when you use programs which are not installed from installers.
> Is there a solution to this problem probably caused by Windows 10?
No.
> Shall return to Windows 8?
No.
Best wishes,
Boris