If you are getting an 'internal server error' when you are trying to rename or remap a certain field in your import mapping, it is likely that you have selected that very same field as the 'Unique ID per item' in the Settings > Project settings menu.
In order to get rid of the error, you can temporarily set the 'Unique ID per item' in the project settings to '-'. Then, you can rename or remap the field you were trying to edit before.
After all the necessary changes are made, go back to Settings > Project settings and set the 'Unique ID per item' back to the desired field.