Getting error 'Some bytes have been replaced with the Unicode substitution character while loading file ... with Unicode (UTF-8)"

Question

I am getting an error when trying to open my VS2015 project:

Some bytes have been replaced with the Unicode substitution character while loading file ... with Unicode (UTF-8)

This seems to be related to one VB file in the project which now displays as a section of normal code followed by gobbledegook further down the file.

Here is the part of the file at which this happens:

timeString = CType(worksheet.Cells(EndTimeCell(0), EndTimeCell(1) + col).Value, String)
endtimeValue = DateTime.FromOADate(timeString).TimeOfDay

'Combine Date And time into a single DateTime
combinedStartDateTime = startdateValue.Add(starttim4F�r2��+�G��� ,j���
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This continues to the end of the file.

Is there any way of resolving this and getting the VB back? It worked ok last week, so no idea what has caused this, unless it is some kind of Windows update that has done it.

I tried 'converting' the offending file to UTF-8 format in Notepad, but this made no difference.