I have the same issue with another theme from CMSMaster, CMS Composer does not work anymore since upgrade to Wordpress 5.6. I tried to deactivate everything except CMS Composer, it is not a solution.
There’s a script not loading, editor.min.css : if you use firefox or chrome and inspect element, you’ll see that in the tab “error”, each time you click on the editor, an error is raised: mixed content https and http is not allowed and editor script is not loading. The fix for this is to modify file wp-content/plugins/cmsms-content-composer/js/jquery.cmsmsComposerLightbox.js and look for “http://” and replace by “https://”.
However it’s not all, there’s still an incompatibility with WP 5.6, I don’t know which one. I noticed since I upgraded to WP 5.6 I’m getting a new error message in Site Health: “Your site could not complete a loopback request” (error 403, looks access denied). I don’t know if there’s a relationship or not, what is sure is an incompatibility between CMS Master Content Composer and WP 5.6.
On the website of https://cmsmasters.net/support/ it is mentioned that themes have been updated but are not available yet on Envato (same message 1 week ago).