Error: Can't Open project -- Quartus II 6.0

If anyone finds this later, this is an error with how the attributes are set with the project files. See following link:

https://www.intel.com/content/www/us/en/support/programmable/articles/000084612.html

To solve this:

1. Open a command shell (Start menu -> Run... -> Enter cmd and press OK).

2. Change to the project directory, i.e. cd <project path> and press Enter.

3. Run the one of the following commands:

attrib -R /S /D *

This should clear the write protection flags for all the files and subdirectories in this project direcotry.

It is not necessary to delete or reinstall any files or directories.