Sonos Cloud API HTTP POST always returns 500 - rest

Question

I'm trying to control my Sonos One (Gen 1) speaker through cloud APIs but all the POST request return 500 Internal Server Error without error message.
I generated an access_token as described in the documentation and was able to obtain information about the 'households' inside my network, finally both the group id and the player id.
I tried to obtain information about the status of the group by performing a GET on the following endpoint and it worked:
https://api.ws.sonos.com/control/api/v1/groups/{{group_id}}/playback
Same with other GET requestes (e.g. groupVolume).
The problem arise when I try to make a POST (e.g. changing the volume or playing an audioClip), it returns 500 Internal Server Error without any body.
For all the requests I am using Postman including as Headers:
Content-Type: application/json
Authorization: Bearer [token]
Content-Length: ??? (automatically inserted by postman)
while for the body of the change volume request I select the raw option and then select JSON(application/json) option.
The body is:
{
"volume": 80
}
The endpoint is the following: https://api.ws.sonos.com/control/api/v1/groups/{{group_id}}/groupVolume
I also tried controlling the player with node (Package) and it works.
Also tried to use Wireshark to see what requests the node-sonos package is performing but it seems it's not using the Cloud APIs.
I expect the player to change volume, but the API doesn't do anything and doesn't return any error message.
EDIT:
The cURL request from POSTMAN is the following:
curl -X POST \
https://api.ws.sonos.com/control/api/v1/players/<playerID>/audioClip \
-H 'authorization: Bearer XXX' \
-H 'cache-control: no-cache' \
-H 'content-type: application/json' \
-H 'postman-token: XXX' \
-d '{
"appId": "edu.myInstitute.myName",
"clipType": "CUSTOM",
"name": "Test",
"streamUrl": "http://...mp3_file_url"
}'