I get raw html when i try to preview running application


please can someone help me out i get this error each time i try to preview one of my application

Error 502 - Bad Gateway .error_content { background: rgba(255, 255, 255, 0.23); padding: 10px; width: 641px; margin: 25px 0; display: none; }
#error-msg {
    display: block;

Error 502 - Bad Gateway

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