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

SyntaxError: Unexpected token 'u', "upstream c"... is not valid JSON
    at async k (worker.mjs:3139:15)
    at async worker.mjs:3198:48
    at async d (worker.mjs:2344:16)
    at async Promise.all (index 0)
    at async Hr (worker.mjs:2381:14)
    at async qc (worker.mjs:2377:271)
    at async Ye3 (worker.mjs:3711:3)
    at async worker.mjs:4317:17