Hangouts Chat Bot "The caller does not have permission", PERMISSION_DENIED - hangouts-chat

Question

I created a chat bot using the JAVA Api, based on the sample code from the documentation,
the Bot uses a service account.
The Bot works fine in one Room, but when I try to use it with a new Room I am experiencing an error.
After adding the Bot to the new Room and trying to post a message I receive
{ "code" : 403, "errors" : [ { "domain" : "global", "message" : "The caller does not have permission", "reason" : "forbidden" } ], "message" : "The caller does not have permission", "status" : "PERMISSION_DENIED" }
Is there any additional step necessary after adding the Bot to a room before posting messages ?
Any help with this problem would be greatly appreciated!
Code used:
HttpTransport httpTransport = GoogleNetHttpTransport.newTrustedTransport();
JsonFactory jsonFactory = new JacksonFactory();
HangoutsChat.Builder builder = new HangoutsChat.Builder(httpTransport, jsonFactory, credentials);
builder.setApplicationName(bot);
HangoutsChat service = builder.build();
ListSpacesResponse response = service.spaces().list().execute();
// Look for space Space _space = null;
while (response.getSpaces() != null && _space == null) {
for(Space s : response.getSpaces()) {
if(s.getDisplayName().equals(space)) {
_space = s;
}
}
if (_space == null && response.getNextPageToken() != null && !response.getNextPageToken().isEmpty()) {
String pageToken = response.getNextPageToken();
response = service.spaces().list().setPageToken(pageToken).execute();
} else {
break;
}
}
if(_space != null) {
Message m = new Message();
byte[] data = {0x0A};
m.setText(text.replace("\\r", "").replace("\\n", new String(data)));
Message r = service.spaces().messages().create(_space.getName(), m).setThreadKey(thread).execute();
Element el = new Element("message");
el.setAttribute("space", r.getSpace().getDisplayName());
el.setAttribute("text", r.getText());
el.setAttribute("thread", r.getThread().getName());
outNode.addContent(el);
} else {
throw new Exception("SendHangouts failed " + "Space " + space + " not found");
}