As of Data Loader v.56, Enable OAuth login from browser is enabled by default. Previously, it was disabled by default in Data Loader Settings.
How: Use these instructions to revert this setting to the v.55 behavior.
- Open Data Loader.
- Click Settings, and select Settings from the dropdown menu.
- Deselect the checkbox next to Enable OAuth login from browser.
- Click OK.
Revert Data Loader Settings to Disable OAuth Login from Browser (salesforce.com)