Update WordPress 5.6

Hi, after updating my website to Wordpress 5.6, each time I view my website and want to go to another page of my website, it appaers a black screen with an "X" on the upper right corner. I can not push the "X" to close the black page. So I have to go back in the browser task bar to get to the last page. I have tested it in Firefox, Opera, Edge and PC/Mobile. Every time the black page with the "X". Could you please help me to disable the black page to appear. Thanks a lot. I use the Theme Chigago (Free Wordpress Theme) R.