500 Unexpected token 'u', "upstream c"... is not valid JSON

SyntaxError: Unexpected token 'u', "upstream c"... is not valid JSON
    at async w (worker.mjs:7557:11)
    at async worker.mjs:7620:57
    at async invokeQRL (worker.mjs:4924:20)
    at async Promise.all (index 0)
    at async renderRoot (worker.mjs:5080:3)
    at async renderSSR (worker.mjs:5076:3)
    at async renderToStream (worker.mjs:6427:3)
    at async worker.mjs:8135:17