A Microsoft file hosting and synchronization service.
Same problem for me happened yesterday when I tried uploading to my Personal Vault from my iPhone. I've tried everything possible, signing out and back in of the app, uninstalling and then reinstalling the app, I've got over 400GB of space left so plenty there and I made sure all permissions are given. Everything is as it should be and I can upload to anywhere else in my OneDrive except for my Personal Vault where it won't let me upload anywhere.
I guess I could say it's nice to see I'm not the only one with this problem but it's shame that it seems to be effecting a lot of people without any real solution or fix to have been put forward yet by Microsoft.