Redmine

We used redmine since 2014.10.

Today, I noticed that I can't view some issues and got this log

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ActionView::Template::Error (undefined method `<' for nil:NilClass):
15: </tr>
16: </thead>
17: <% previous_group = false >
18: <tbody>
19: < issue_list(issues) do |issue, level| ->
20: < if @query.grouped? && (group = @query.group_by_column.value(issue)) != previous_group >
21: < reset_cycle %>
app/helpers/issues_helper.rb:26:in `block in issue_list'
app/helpers/issues_helper.rb:25:in `each'
app/helpers/issues_helper.rb:25:in `issue_list'
app/views/issues/_list.html.erb:18:in `block in app_views_issues_list_html_erb___2247383097831647094_64419180'
app/views/issues/_list.html.erb:1:in `_app_views_issues__list_html_erb___2247383097831647094_64419180'
app/views/issues/index.html.erb:63:in `_app_views_issues_index_html_erb__3439583084481548753_69925830795880'
app/controllers/issues_controller.rb:87:in `block (2 levels) in index'
app/controllers/issues_controller.rb:86:in `index'
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I'm not engineer about Web, Rails.. Ruby..
So, I don't know how can I access this problem.

Please help me.
Thanks.