File cannot be modified

I have been working on an Unreal Engine project for several months. Today, a .cpp file that I edit frequently became impossible to edit in Rider. If I make changes to the file, then click away from Rider, I get the following error:

'Cannot Save Files - Cannot save <filename and path>. Unable to open the file for writing.'

If I try to hit the Lock button in the bottom right corner, I get the error message:

‘Error - Failed to change read only flag for <filename and path>’

The file has been fine to work with up until this point. I can still modify the file with another text editor, such as Notepad. The file is not marked as Read-Only by Windows. I am logged into a user that has Read and Write access to the file. I have also tried disabling my anti-virus software and restarting my machine. The problem does not occur with any other files in the project.