Ran into the same error yesterday (Project Failed To Load) on MPC Keys. I've been saving a song that I was working on to a thumb drive, and overriding each iteration as I went. The last save must not have processed properly, and I got this error.
Here's how I recovered the project.
Put the thumb drive into my computer, either Mac or PC, you have to locate the project folder for that particular project, it should contain all of your samples, and several other settings applicable to your project.
Copy that folder to your desktop and then back onto the thumb drive so that it's visible when you plug the thumb, drive back into your MPC.
Once you're back on your MPC, navigate to that folder, and load everything that is not a sample. If the MPC says this already exists, choose the override and load it anyway.
Eventually, you will have loaded all of your sequences, all of your drum sample programs, and whatever other miscellaneous things that made up that project.
This worked for me and I was able to recover my entire song, and then resave it. I'm sure this was already mentioned somewhere on the forums, but hopefully this helps somebody