Helo,
my xpath does validate in firePath but when I try to send _key I get an error.
userID = driver.find_elements_by_xpath(\".//*[@id=\'UserName\']\
This error occurs when one tries to perform action on list instead of element. I was getting similar error when I was trying to click on button to submit credentials. I found the work around by emulating pressing enter key on keyboard. e.g. find any element on page using xpath/css, etc. and send enter key.
driver.find_element_by_id('test_id').send_keys(Keys.ENTER)