Very sorry for the confusion.
I .. wrongly.. assumed that Excel when I double click an Excel sheet and another one Excel would run twice, i.e. in Windows taskmanager under tab Details, there would be 2 Excel.exe running there.
Just opened a few Excel sheets (workbooks) - double clicking the Excel sheet.
The Windows taskbar shows (in this case) 4x Excel-instances. However upon checking, only 1 Excel.exe is running.
These 2-3 or more Excel's in the taskbar brought me to wrongly assume multiple Excel.exe would be running.
Sorry again.
Anyway, that said, the issue is with having multiple workbooks open then.
Usually I have to close all workbooks, keeping open the one that I am and was actively working on that.
However, in rare cases I need to close all the workbooks and relaunch Excel to get this 'fixed' (in a miraculous way)
There were a few threads in the past, like:
-Paste is not working in one file only
-Thread: copy and paste not working between workbooks
But regretfully they didn't bring me any further.