In my case I forgot to set config value ture for use secur admin url.
I read in your statement, you mentioned that SSL is installed, Please check if admin url is being opened as secure ( using https:// ). If your website is running on HTTPS then admin url should be set to use secure url. you can set config value for admin url
php bin/magento config:set web/...
Hello you should use the event:
<observer name="Observer" instance="Vendor\Module\Observer\Observer" shared="false" />