EDIT: This seems to have been a problem with the Firefox RESTClient extension. Accessing the URI from Java did work (cf. my answer). So the problem is solved for me. /EDIT

I'm trying to create a comment for a work item on the RTC cloud trial (https://jazz.net/sandbox01-ccm), using the OSLC API, following https://jazz.net/wiki/bin/view/Main/WorkItemAPIsForOSLCCM20#Adding_comments.

What I did: I sent a POST request to the URI "https://jazz.net/sandbox01-ccm/oslc/workitems/[...]/rtc_cm:comments/oslc:comment" with the following body
{
"dcterms:description": "test comment"
}

What happened: I got the response status code "403 Permission denied" and the message "Your account does not have the group memberships required to access the requested resource.". However, I'm an administrator for this project and in the settings I gave myself all the permissions I could find.

Do you have any ideas why that might be? (I'm new to RTC, so I may be missing something obvious.)