I loaded PHP 8 onto one of my test machines and found the source of your problem. As I suspected, it is not with CKGEdit.
PHP Fatal error: Array and string offset access syntax with curly braces is no longer
supported in . . ./vendor/marcusschwarz/lesserphp/lessc.inc.php on line 761
PHP Fatal error: Array and string offset access syntax with curly braces is no longer
supported in . ./vendor/marcusschwarz/lesserphp/lessc.inc.php on line 2762
The errors occur in the Release Candidates, not in the just released stable. You have to update your Igor.