I recently tried to import contacts into Gmail and encountered the cryptic (and completely useless) error message “Oops. An unknown error occurred while importing your contacts”.
Turns out Gmail chokes if it encounters a duplicate email address during the contact import process. It also appears this error only occurs with the latest version of Gmail. Therefore, a nice and easy workaround is to switch to the “Older version” of Gmail by clicking the “Older version” link (top right corner), import the contacts, then switch back to the newer version of Gmail.
Worked for me.