The same even happens if I create a new file with ANSI encoding in Notepad++ and enter some umlaute.
When I save and reload from disk, the encoding changes to Windows-1255 and the umlaute are rubbish.

About Mackintosh
I just turn off ‘auto encoding character’ option.

I’m a little bit confused by the video and the new text.
Video shows the previous question about privileges
whereas the new text indicates an encoding issue.
So I assume you do have two different issue.

Concerning the video let me try to explain what happens (or what doesn’t happen)
With introduction of UAC Microsoft changed their security concept and allowed
that different process with different privilege levels run on the desktop. This means,
that if you logon as a normal user explorer.exe gets started with the security token
of this normal user. When you start npp as administrator this process gets elevated rights,
meaning different security token.
The desktop where your file is located belongs to the explorer.exe process - if you drag this file
(as normal user) into npp (which run elevated) the security tokens don’t match and therefore it gets
rejected. You can only open the file if you use the file dialog menu from npp or when starting another
instance from explorer as administrator and dragging the file from it.
If you want to get more details about how exactly this works I would recommend searching the web for

• User account control (UAC)
• Mandatory Integrity Control (MIC) and
• User Interface Privilege Isolation (UIPI)

About the encoding - yes this can happen - there is no guarantee that npp finds the “correct” codec.
I tried to explain this already here.

Cheers
Claudia

Some files are automatically defined as Mackintosh instead of the Windows-1251.
The problem occurs only when using the Cyrillic alphabet.

I assume that the access rights from source to destination are different.
E.g. when starting explorer with a normal user account and you want to drag
something into an application with evaluated rights than it get rejected
as otherwise you would cancel the whole permission concept.

Cheers
Claudia

Every new version shows the changes whens started first.
The file is located in the installation directory and is called change.log.

Concerning your issue - you can open an issue here.

Cheers
Claudia

Settings/Preferences/Default Directory: Use new style save dialog

Reverts to checked when program restarted

This was OK in version 7.3.0

Having the same issue. :( Should I just use 7.3.0 instead? What does the new version offer any ways?

Did you check if settings are still set.
Settings->preferences->backup
Remember current session …
and
enable session snapshot …

Cheers
Claudia

Reverts to checked when program restarted

This was OK in version 7.3.0

I would also mension that the Uninstaller should also delete that Folder as someone choose to not Keep custom Settings. But ist just a Suggestion.

Thanks donho if it was something you changed in 7.3.1 in compared to 7.3.