@egon0 Either way that isn't an error, just a warning for sloppy code. The result will be that $attachment['thumb_url'] just gets set to null (which will likely be tested later in the javascript, so nothing is broken - just erroneously coded).
It could be patched either by always supplying a value for the index 'thumb_url' from !GNUsocial or doing an array_key_exists test in !qvitter
I'm not immediately sure if it should just be patched or perhaps rewritten a little bit :)