ICO of the executable file

[RhoSigma]

... the icon displayed in the executable file window is the right one; however, the icon of the executable file is even "old" and belonged to a project I haven't worked on for many months ...

So if I get that correctly, only the icon shown in the file list of the Windows Explorer is wrong? But program window and Taskbar icons are correct?

Then I'd suspect it's a problem within Explorer's icon cache. That's a know issue since many many years, I noticed it first on Win7, but already had it on Win10 too.

Switch Explorer to view hidden files, then navigate into folder %localappdata% (you can paste that into the Explorer address line) and there delete the file "IconCache.db", after that close everything and restart your system.