If you have configured some background accelerators(e,g. Metadata Inheritance or Calculated Expressions, etc) on adding item to the library, you may face errors during adding files via Upload button.
To prevent that you need to turn on "requires check-out" setting for the document library. You can do that following these steps:
- Open Library settings
- Select "Versioning Settings"
- Select "Yes" radio button for the "Require Check Out" and "OK" button