“The project “XYZ” could not be saved.”
After I close the dialog, and save as, it saves just fine. What is wrong? Hard drive issue? The issue seems to be intermittent, but more common after Cubase has been open for a few ours and has been creating several .bak files.
You probably didn’t initially save the project and just started working on the project with the dummy name ‘untitled…’ or so? A good workflow habit is to first name your project folder, setup you project and first save it with the same name inside that folder before you actually start to work on it. Now start working on your project and you can save it on any desired moment. Also your *.bak files will now correspond to the same name.
Or the file you opened was read only.
Here’s a photo of the error message I get occasionally.
I’ve seen this before in the past and if I remember correctly it had something to do with RAM? Seeing you have 64 GB it’s certainly not insufficient memory? 
What could happen is if you let windows handle your swap file automatically it might create a massive 96 GB file. The chance of hard page faults increases significantly. As a result Cubase can’t match the project currently stored in memory with the one saved earlier to disk and offers the option to save it under another name. Especially with larger projects it’s more likely to happen.
If you have not manually adjusted your swap file settings you could try to turn it off completely but then you can’t produce system memory dumps anymore. If you still want that option you could set to 4096 MB or so, that is sufficient. But I have it disabled permanently because I next to never get BSOD’s. If you still get errors you might have to test your memory and see if it’s reliable. But lets hope it doesn’t have to come to that?