error handling json: Error: lexical error: invalid char in json text.

When running some of my R scripts, I always get following errors which shuts down the server.
Cannot figure out what json text it is referring to, and what causes it.

[2021-08-09 16:03:38.041] error handling json:  Error: lexical error: invalid char in json text.
                                       Content-Length: 10492    {"json
                     (right here) ------^

Stack trace:
1: parse_string(txt, bigint_as_char)
2: parseJSON(txt, bigint_as_char)
3: parse_and_simplify(txt = txt, simplifyVector = simplifyVector, 
    simplifyDataFrame = simplifyDataFrame, simplifyMatrix = simplifyMatrix, 
    flatten = flatten, ...)
4: jsonlite::fromJSON(data, simplifyVector = FALSE)

[2021-08-09 16:03:38.046] Error: Unexpected non-empty line
Call: self$read_header()
Stack trace:
1: stop("Unexpected non-empty line")
2: self$read_header()
3: self$fetch(blocking = FALSE)

[2021-08-09 16:03:38.046] exiting

[Error - 4:03:38 pm] Connection to server is erroring. Shutting down server.
[Error - 4:03:38 pm] Connection to server is erroring. Shutting down server.
[Error - 4:03:38 pm] Connection to server is erroring. Shutting down server.

Start with the reported stack trace, especially jsonlite::fromJSON(data, simplifyVector = FALSE), parseJSON, and self$read_header(). Reproduce the failure using the affected R scripts or equivalent input and determine which message contains the invalid JSON. Done should include a confirmed cause and a way for the server to handle it without shutting down.