I'm trying to use Data Loader to import a group of opportunities. Everytime I run the Data Loader it only imports the first record. All the other records fail with the message "An unexpected error occurred during the import of the following row: 'External Unique Id: xxxxxxx'". After running the Data Loader, I can modify the Data file and remove the first record that was imported. By running the Data Loader again, the first row (previously the second row) will import successfully.
Any idea what could be causing this behavior?