You will need another route for this info:
https://<portalUrl>/portal/sharing/rest/content/users/<userName>/folderId/items/<itemId>
You can find this by logging in to the Community Root:
https://<portalUrl>/portal/sharing/rest/community
The follow Self, User Content, select folder, select item and look for Sharing Groups on the bottom of the page.
Thank you for the reply but I think I am doing sth wrong. My link looks something like that

and it does not work. I got and error
<b>Error: </b>400<br/><br/>
I think you forgot to replace the folderId.
You can use the browser to access
https://<portalUrl>/portal/sharing/rest/community
and go from there.
I have used this request
https://maps-eu.xxx.com//portal/sharing/rest/content/users/MPOD@xxx.com/items/@Value(itemId)/sharing?f=pjson
and got this info:
"sharing": {
"access": "shared",
"groups": g
"035d3b1262bb4dddfdfd9a0a260e2c4d76e4",
"ce750fea888a4ddfdfdfddfde6065536adfa31"
How can I find the actual name of the group?