I just had this exact problem and I believe we found the source of it. We were also using the DELETE method to reach out to an endpoint which returned a response body of null. The call was working as expected until we changed the sys_properties record for glide.outbound_http_log.override to true AND the property glide.outbound_http_log.override.level to all this cause the error you mention as well as failing to insert the call into the outbound http log BUT the call still worked as expected on the 3rd party system.

It seems to me the source of this problem comes from ServiceNow being unable to handle logging a REST response from a DELETE method where the response body is null .

I hope this saves some folks some time when they come across this, I also hope ServiceNow fixes this bug so that we can start capturing outbound http logs at a level other than basic without breaking requests that match the above criteria.