It seems this exception appeared after schema change 21 (http://blog.musicbrainz.org/2014/09/17/2014-11-17-schema-change-release-details/):

D: 09:39:40 GET https://musicbrainz.org/ws/2/collection
Traceback (most recent call last):
File "./picard/webservice.py", line 287, in _process_reply
handler(document, reply, error)
File "./picard/collection.py", line 113, in request_finished
user_collections[node.id] = Collection(node.id, node.name[0].text, node.release_list[0].count)
File "./picard/webservice.py", line 95, in _getattr_
raise AttributeError(name)
AttributeError: release_list
D: 09:39:41 Received reply for https://musicbrainz.org/ws/2/collection: HTTP 200 (OK)

It affects 1.3 (and prolly previous releases).