Setting Membership entity as Process Manager generates exception

Hi everybody,

I use Bonita 7.0.1 and have a process "Process1" deployed and would like to set a process manager.
I have a role "ProcessAdmin" and a group "Group1".
I can set any user, group or role as manager and it works fine

But when setting "ProcessAdmin of Group1" as Process Manager, nothing happens and an exception is present in the logs:
*
GRAVE: The API "bpm#processSupervisor" has encountered an unknown error
org.bonitasoft.web.toolkit.client.common.exception.api.APIException: The API "bpm#processSupervisor" has encountered an unknown error
at org.bonitasoft.web.rest.server.engineclient.GroupEngineClient.get(GroupEngineClient.java:53)
at org.bonitasoft.web.rest.server.datastore.organization.GroupDatastore.get(GroupDatastore.java:90)
at com.bonitasoft.web.rest.server.api.bpm.process.APIProcessSupervisor.fillDeploys(APIProcessSupervisor.java:98)
at com.bonitasoft.web.rest.server.api.bpm.process.APIProcessSupervisor.fillDeploys(APIProcessSupervisor.java:39)
at org.bonitasoft.web.rest.server.framework.API.runSearch(API.java:258)
at org.bonitasoft.web.rest.server.framework.APIServletCall.doGet(APIServletCall.java:181)
at org.bonitasoft.web.toolkit.server.servlet.ToolkitHttpServlet.doGet(ToolkitHttpServlet.java:183)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:620)
at org.bonitasoft.web.toolkit.server.servlet.ToolkitHttpServlet.service(ToolkitHttpServlet.java:75)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:727)
at org.bonitasoft.web.toolkit.server.servlet.ToolkitHttpServlet.service(ToolkitHttpServlet.java:226)*

I have the same behaviour on 3 different machines.

Does anyone have the same thing? Any idea if this is a bug?
Thank you for any answer you may have!

Jean-Yves