HI,
Here's the general information on trouble shooting VEC issues. Troubleshooting the Visual Experience Composer and Enhanced Experience Composer
However, I believe your issue seems specific to X-Frame-Options.
To overcome this could you please try turnin OFF the EEC, (To disable the Enhanced Experience Composer, go to Setup > Preferences and turn off the Enable Enhanced Experience Composer option.)
And then do the following, use a browser extension like Requestly (Chrome or Firefox) or Modify Response Headers (Firefox) that can overwrite the X-Frames header options for your site and allow them to be loaded in iFrames, enabling the VEC.
To use the Requestly extension on Chrome or Firefox:
- Turn off the Enhanced Experienced Composer.
- Install the Requestly browser extension on Chrome or Firefox.
- Open the extension and configure it using the following:
- Select Modify headers.
- Enter the following:
- Rule name
- Modification rules
- Toggle Add to Remove.
- Toggle Request to Response.
- Enter "X-Frame-Options" as the header name.
- Repeat previous steps and enter "x-frame-options" as the header name.
Note: Headers that are manipulated via Requestly are case sensitive.
- Change Equals to Contains as the condition for the source URL and enter the URL of the activity that you are trying to load in the VEC.

- Click Save.
You should now be able to load the page quickly with the Visual Experience Composer.