"Sorry, an error has occurred" when saving user preferences

Bug
7 
HTTP_HOST: forum.kabissa.org
REQUEST_URI: /users/tobiaseigen
REQUEST_METHOD: PUT
HTTP_USER_AGENT: Mozilla/5.0 (Macintosh; Intel Mac OS X 10_10_2) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/40.0.2214.94 Safari/537.36
HTTP_ACCEPT: */*
HTTP_REFERER: https://forum.kabissa.org/users/tobiaseigen/preferences
HTTP_X_FORWARDED_FOR: 73.35.188.188, 199.27.133.71
HTTP_X_REAL_IP: 199.27.133.71
username: tobiaseigen

params:
  auto_track_topics_after_msecs: 240000
  bio_raw: Founder of Kabissa. Community builder. Developer. Technology guide to civil society and grassroots or
  website: http://linkedin.com/in/tobiaseigen
  location: Near Seattle on the west coast of USA
  name: Tobias Eigen
  locale: 
  email_digests: true
  email_direct: true
  email_always: false
  email_private_messages: true
  dynamic_favicon: true
  digest_after_days: 1
  new_topic_duration_minutes: 2880
  external_links_in_new_tab: true
  mailing_list_mode: false
  enable_quoting: true
  disable_jump_reply: false
  custom_fields: 
  user_fields: null
  muted_category_ids: 5
  watched_category_ids: 1,19,20,18,12,6,7,13
  tracked_category_ids: -1