Can't "Save as default"

I’ve noticed that my defaults have been reseted for some unexplained reason (for example, my default page size is now “letter” instead of A4),
so I’ve opened one of my recent files and clicked “save as default” on every option.
even after that, when I open a new file, my page size is still “letter”.
thanks for any help,

Check what the value of ‘Default paper size type’ is set to, on the General page of Preferences.