Fix endpoint

app/assets/javascripts/labels_select.js.coffee

@@ -148,6 +148,7 @@ class @LabelsSelect
         $.ajax(
           type: 'PUT'
           url: issueURL
+          dataType: 'JSON'
           data: data
         ).done (data) ->
           $loading.fadeOut()

app/assets/javascripts/users_select.js.coffee

@@ -94,6 +94,7 @@ class @UsersSelect
               .fadeIn()
             $.ajax(
               type: 'PUT'
+              dataType: 'json'
               url: issueURL
               data: data
             ).done (data) ->

app/views/shared/issuable/_sidebar.html.haml

@@ -2,9 +2,9 @@
   .issuable-sidebar
     .block.issuable-sidebar-header
       - if issuable.to_ability_name == 'merge_request'
-        - issuable_url = namespace_project_merge_request_path(@project.namespace, @project, issuable.id, :json)
+        - issuable_url = namespace_project_merge_request_path(@project.namespace, @project, issuable.iid)
       - else
-        - issuable_url = namespace_project_issue_path(@project.namespace, @project, issuable.id, :json)
+        - issuable_url = namespace_project_issue_path(@project.namespace, @project, issuable.iid)
       %span.issuable-count.hide-collapsed.pull-left
         = issuable.iid
         of
@@ -93,7 +93,7 @@
               .light None
           .selectbox.hide-collapsed
             - issuable.labels.each do |label|
-              = hidden_field_tag "{issuable.to_ability_name}[label_names][]", label.id, id: nil
+              = hidden_field_tag "#{issuable.to_ability_name}[label_names][]", label.id, id: nil
             .dropdown
               %button.dropdown-menu-toggle.js-label-select.js-multiselect{type: "button", data: {toggle: "dropdown", field_name: "#{issuable.to_ability_name}[label_names][]", ability_name: issuable.to_ability_name, show_no: "true", show_any: "true", selected: issuable.label_names.join(","), project_id: (@project.id if @project), issue_update: issuable_url, labels: (namespace_project_labels_path(@project.namespace, @project, :json) if @project)}}
                 %span.dropdown-toggle-text