What about other projects - do they behave similarly? What happens if you attempt to export a later portion of the project - cutting out a few minutes in the middle, placing them on a new track and exporting?
Can recollect no instance where a user was unable to save a portable project.
Considering all the diagnostic steps that you have taken, one might conclude that either the project is corrupt or something has gone awry with your PC.
Try the following...
Save your project using FILE|SAVE PROJECT AS to the same folder that contains the source files.
With Windows Explorer (the PC file manager), open that folder and zip ALL the files that it contains.
Upload that zip file to a server and share a public link to it here.
In Windows 10 it's easy to create a zip file. Open the folder containing the files and click anywhere on the right side, where the files are shown.
Press <ctrl-A> to select (highlight) all files. Right-click on any file and SEND TO|COMPRESSED (zipped) FOLDER.
The result will be a file with a .zip extension that has the same name as the file you right-clicked. Upload that file.