Issue adding community members via API (SDK) - HTTP 403 Forbidden

[csandres]

When we try to add community members programmatically (Social Business Toolkit), we get 403 Forbidden HTTP/1.1:

[3/15/18 12:01:14:852 CET] 0000004e SystemErr R Caused by: com.ibm.sbt.services.client.ClientServicesException: Request to url https://myConnectionsURL.com/communities/service/atom/community/members?communityUuid=41906d21-45fd-497a-a7c2-c0183afd678a returned an error response 403:Forbidden HTTP/1.1 403 Forbidden [Date: Thu, 15 Mar 2018 11:01:14 GMT, X-Powered-By: Servlet/3.0, Expires: Thu, 1 Jan 1970 00:00:00 GMT, Cache-Control: no-store, no-cache, must-revalidate, X-LConn-Auth: true, Set-Cookie: lcLang=en_us; Expires=Tue, 14-Mar-28 21:08:54 GMT; Path=/, X-UA-Compatible: IE=Edge, Keep-Alive: timeout=10, max=100, Connection: Keep-Alive, Transfer-Encoding: chunked, Content-Type: text/xml; charset=utf-8, Content-Language: en-US] org.apache.http.conn.BasicManagedEntity@bbbe5672
[3/15/18 12:01:14:853 CET] 0000004e SystemErr R at com.ibm.sbt.services.client.ClientService.throwClientServicesException(ClientService.java:1347)
[3/15/18 12:01:14:853 CET] 0000004e SystemErr R at com.ibm.sbt.services.client.ClientService.findErrorHandler(ClientService.java:785)
[3/15/18 12:01:14:854 CET] 0000004e SystemErr R at com.ibm.sbt.services.client.ClientService.findHandler(ClientService.java:1305)
[3/15/18 12:01:14:854 CET] 0000004e SystemErr R at com.ibm.sbt.services.client.ClientService.processResponse(ClientService.java:1130)
[3/15/18 12:01:14:854 CET] 0000004e SystemErr R at com.ibm.sbt.services.client.ClientService._xhr(ClientService.java:1038)
[3/15/18 12:01:14:854 CET] 0000004e SystemErr R at com.ibm.sbt.services.client.ClientService.execRequest(ClientService.java:1003)
[3/15/18 12:01:14:854 CET] 0000004e SystemErr R at com.ibm.sbt.services.client.ClientService.xhr(ClientService.java:966)
[3/15/18 12:01:14:854 CET] 0000004e SystemErr R at com.ibm.sbt.services.client.ClientService.post(ClientService.java:871)
[3/15/18 12:01:14:854 CET] 0000004e SystemErr R at com.ibm.sbt.services.client.ClientService.post(ClientService.java:867)
[3/15/18 12:01:14:855 CET] 0000004e SystemErr R at com.ibm.sbt.services.client.base.BaseService.createData(BaseService.java:199)
[3/15/18 12:01:14:855 CET] 0000004e SystemErr R at com.ibm.sbt.services.client.base.BaseService.createData(BaseService.java:183)
[3/15/18 12:01:14:855 CET] 0000004e SystemErr R at com.ibm.sbt.services.client.base.BaseService.createData(BaseService.java:216)
[3/15/18 12:01:14:855 CET] 0000004e SystemErr R at com.ibm.sbt.services.client.base.BaseService.createData(BaseService.java:231)
[3/15/18 12:01:14:855 CET] 0000004e SystemErr R at com.ibm.sbt.services.client.connections.communities.CommunityService.addMember(CommunityService.java:826)
[3/15/18 12:01:14:855 CET] 0000004e SystemErr R ... 33 more

According to the logs the user is authenticated and authorized:

[3/15/18 12:01:14:852 CEST] 00000039 WSCredentialI > isUnauthenticated Entry
[3/15/18 12:01:14:852 CEST] 00000039 WSCredentialI < isUnauthenticated Exit
false

Thanks in advance.
Regards.