I am also having a problem with captcha not displaying on the Oxygen theme. If I try opening the image as a web page, it displays:
Undefined index: activation_number
The following notice has stopped further execution:
Undefined index: activation_number in /home/bugs/core/modules/main/controllers/Main.php, line 4741
Stack trace:
require()
/home/bugs/core/framework/Context.php, line 316
thebuggenie\core\modules\main\controllers\Main->runCaptcha()
/home/bugs/core/framework/Context.php, line 2383
thebuggenie\core\framework\Context::performAction()
/home/bugs/core/framework/Context.php, line 2679
thebuggenie\core\framework\Context::go()
/home/bugs/public/index.php, line 29