HTTP Post multipart/form-data: Invalid Content-Disposition value - http

Question

I am working on a HTTPS POST request which contains two multipart/form-data entries.
But for whatever reason I can't get it working.
This is the request I am sending:
POST /my/api/endpoint HTTP/1.0
Host: myserver.de
Content-Type: multipart/form-data; boundary=123456
Content-Length: 147
Connection: close
X-API-KEY: 123
--123456
Content-Disposition: form-data; name="edf"
EDF
--123456
Content-Disposition: form-data; name="parameters"
PARAMETERS
--123456--
What is not shown above that the string is null-terminated ('\0' after --123456--).
But the response I get is:
HTTP/1.1 400 Bad Request
Server: nginx/1.21.3
Date: Wed, 04 May 2022 06:35:34 GMT
Content-Type: application/json; charset=utf-8
Connection: close
{"type":"https://tools.ietf.org/html/rfc7231#section-6.5.1","title":"One or more validation errors occurred.","status":400,"traceId":"00-0c7bdc085f3f2543aee5f677554f0568-3911d802790ef040-00","errors":{"":["Failed to read the request form. Form section has invalid Content-Disposition value: "]}}
Is someone able to determine what I am doing wrong here?
Edit:
If I remove the null termination I get
HTTP/1.1 400 Bad Request
Server: nginx/1.21.3
Date: Wed, 04 May 2022 06:44:43 GMT
Content-Type: application/json; charset=utf-8
Transfer-Encoding: chunked
Connection: close
122
{"type":"https://tools.ietf.org/html/rfc7231#section-6.5.1","title":"One or more validation errors occurred.","status":400,"traceId":"00-504d94c952c60246b96667d6a8c2e690-d160644f50acf14a-00","errors":{"edf":["The EDF field is required."],"parameters":["The Parameters field is required."]}}
0
Do you notice the 122 and 0 numbers - where are they comming from?