We came across an issue where we were getting an error when calling the endpoint "/_api/v1.0/files/root/children" on a SharePoint site in Office 365.
There are no additional details in the error message.
The fix for this error is very simple however the error message itself is not very friendly.
The endpoint gets the documents from the default document library and we will see this error if the default document library is deleted.
In most cases this will be a library named Documents.
Recreating this library will fix this error.