There are various reasons why the browser extension for Chrome and Firefox may not be able to access local storage. When this happens, you'll see an error when you try to send or receive encrypted emails.
First thing to check is your browser cookies settings.
Chrome users: chrome-extension://bnjglocicdkmhmoohhfkfkbbkejdhdgc/chrome/texts/chrome_content_settings.htm
Firefox users: moz-extension://c41c9281-6b09-4669-9098-e7e71f26b0cf/chrome/texts/firefox_content_settings.htm
Copy the appropriate link, paste it into your browser, and follow the outlined steps and see if the situation improves. This fixes it for most people.
If following the steps didn't fix it for you, please write me at firstname.lastname@example.org