I've been running PHP 7.4.29 from install, but have been getting the same issue. VisualEditor works flawlessly when creating pages, but throws a 500 error when editing existing ones. I tried adding $wgGroupPermissions['*']['writeapi'] = true; at the end of my LocalSettings.php at one point and thought it was fixed because I was able to edit a page at first, but immediately after moving off that page VisualEditor refused to work again, on either the Main Page or the page I had just tried to edit.