It seems like the server is blocking the access to the specific page .
this is working in my site .
http://prntscr.com/o6zt76
Please check if your site url and wordpress url are same in wp-admin -> settings -> general .
Secondly please contact your webhost if there is any server mod_security enabled to restrict the posting and receiving of html data .
Thirdly if there is any third party security plugin please try to deactivate it .
At last to debug further please share your site url , admin credentials in private reply to check this issue at your end .