Steps to reproduce the issue

Another Ajax bug on admin login

Load the /administrator/ page.

Edit /administrator/index.php and place a die(); after the opening <?php - doing this replicates "bug" on your site, it could be a php warning, or an exception in a plugin, or any PHP error. Most common is a plugin outputting PHP Notices or warnings...

Try to login

Note that you get ZERO FEEDBACK now, no error messages are shown. You try to click Login In again and no feedback... you are now at a loss as why you cannot login.

This is because the JSON response to the AJAX request is malformed and Joomla's JS doesn't handle it with a try/catch block

Expected result

Visual "there was a problem" kind of message... JLIB_JS_AJAX_ERROR_OTHER even :)

Actual result

No feedback - except a console.log error message

SyntaxError: JSON Parse error: Unexpected EOF

You can change the die() statment - lets output some text, make your /administrator/index.php be this now <?php arghhhh and you will now get the more common error:

SyntaxError: JSON Parse error: Unrecognized token '<'

Additional Information

There was a PR for [4.0] Remove AJAX for login #29571 ... which would make this issue null and void by removing ajax