The visual experience composer works for me local page. However, when I try to browse any server website, there are errors like: Refused to display 'the url' in a frame because it set 'X-Frame-Options' to 'sameorigin'.
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:
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.