What version of runtime are you using?

Some where it has stopped working are 9.2.4 and some that are working 10.0.1

If it is the runtime why would it have worked OK previously?

I was thinking that it was an issue that was fixed in 10.4.0 but that was relating to HTTPGET when I checked.

Have you tried inspecting the response via Wireshark (or something similar), before it reaches the COBOL program, to see if the server is actually sending back the correct response?

You could also try sending the request via another tool like Insomnia to see if the same happens there.

Is anything logged on the server that could explain the null response?

You could try increase the timeout values for the RMNET. Maybe the request is taking too long on some occasions and causing a timeout.

HttpSetConnectTimeout - https://bit.ly/3MBZuEh

HttpSetTimeout - https://bit.ly/3LnQ4fe

The tool that is the most direct analog to RMNet is the curl command line tool.  This is due to the fact that RMNet uses libcurl.  Using curl might help you diagnose the issue more easily, and a failing curl example would be easier if you seek technical support from the web service vendor.

Do you know the value of response-status when the response-payload is null?

The response status is 7

I converted the post into a curl command which I think is correct i.e.

curl.exe --trace trace.txt -k POST -H "multipart/form-data; boundary=12345" --data "--12345/r/n'Content-Disposition: form-data; name=uid/r/n/r/nUSERNAME/r/n--12345/r/n'Content-Disposition: form-data; name=pwd/r/n/r/nPASSWORD/r/n--12345/r/nContent-Disposition: form-data; name=uploadfile; filename=DISKFILENAME/r/nContent-Type: multipart/form-data/r/n/r/nxml-content/r/n--12345--" URL

When I run it gives the following error

curl: (6) Could not resolve host: POST
<?xml version="1.0" encoding="UTF-8"?>
<Error Message="The http(s) request should be multipart encoded." />

If the response status is 7, then for some reason the request is being blocked, either on the client machine, or at the server.  

Do a search on "curl error code 7" and see if any of the suggestions help.

As far as the command line, I would place the request payload data in a file.  That will make the command line much more manageable.

I seem to have got a bit further trying the upload with cURL but it is now giving me the following error

curl: (6) Could not resolve host: POST
<?xml version="1.0" encoding="UTF-8"?>
<DMSSubmissionReport>
<Error Message="XML error: Error on line 1: Content is not allowed in prolog." />
</DMSSubmissionReport>

I have checked the XML upload file and it all looks intact and in fact uploaded via a web page loader in chrome successfully

Any thoughts?

Error 6 seems like a DNS issue on your client. But then you get a response from the server too? So it actually has resolved the domain to IP?

The XML error looks like the same issue discussed here - stackoverflow.com/.../fatal-error-11-content-is-not-allowed-in-prolog