Browsers are blocking a frame with origin of its own domain

Question

At this site: https://mdintro.com/registration/?user_group_id=Employer I'm getting several errors that block users from entering text on the description field. Chrome and Safari shows this error:

Uncaught DOMException: Blocked a frame with origin "https://mdintro.com" from accessing a cross-origin frame." fckeditorcode_gecko.js:36 ...

But why is it blocking a frame that come from the same domain?

Firefox (that probably has other criteria) is not blocking this frame but it gives the following warning on FCKeditor:

"Synchronous XMLHttpRequest on the main thread is deprecated because of its detrimental effects to the end user’s experience. For more help http://xhr.spec.whatwg.org/ fckeditorcode_gecko.js:67:105"

Could someone please kindly help me!?

UPDATE: This site has a SmartJobBoard Admin Panel that uses the same FCKeditor frames and it results with no errors. Chrome doesn't show any errors on the console either and the admin user can enter enter text on the text editor. They are both referencing to the same FCKeditor extension.

Here's the frame shown on https://mdintro.com/registration/?user_group_id=Employer

<!-- USER: REGISTRATION (CONSOLE ERROR: BLOCKED) -->
<iframe 
  id="CompanyDescription___Frame" 
  src="https://mdintro.com/system/ext/fckeditor/editor/fckeditor.html?InstanceName=default_value&amp;Toolbar=BasicAdmin" 
  width="345px" height="200" frameborder="0" scrolling="no" 
  style="margin: 0px; padding: 0px; border: 0px; background-color: transparent; background-image: none; width: 345px; height: 200px;">
</iframe>

Here's the frame shown on inside the SmartJobBoard Admin Panel:

<!-- ADMIN: EDIT JOB POSTING (NO ERRORS) -->
<iframe 
  id="default_value___Frame" 
  src="https://mdintro.com/admin/../system/ext/fckeditor/editor/fckeditor.html?InstanceName=default_value&amp;Toolbar=BasicAdmin" 
  width="100%" height="200" frameborder="0" scrolling="no" 
  style="margin: 0px; padding: 0px; border: 0px; background-color: transparent; background-image: none; width: 100%; height: 200px;">
</iframe>

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SOLVED: I applied tdebroc's solution from: Blocked a frame with origin "<mydomain>" from accessing a frame with origin "https://www.facebook.com"