When you create a filesystem, there is a parameter named as "block percent free". This parameter should be "5%", so a 5% of your partition size can only be written by the "root" user.
You can decrease this value or just free some space. You can try to create files or folders as root as well.
Buddy, this message was posted 6 month ago, when this issue was a thing. Does not applied nowdays. But thank you for your help.