Redmine

In a certain project I get this instead of the issuepage:

Internal error

An error occurred on the page you were trying to access.
If you continue to experience problems please contact your redMine administrator for assistance.

At the server, I see this error:

ActionView::TemplateError (undefined method `-' for nil:NilClass) on line #16 of app/views/issues/_list.rhtml:
13: </tr></thead>
14: <% previous_group = false >
15: <tbody>
16: < issue_list(issues) do |issue, level| ->
17: < if @query.grouped? && (group = @query.group_by_column.value(issue)) != previous_group >
18: < reset_cycle %>
19: <tr class="group open">

app/helpers/issues_helper.rb:28:in `issue_list'
    app/helpers/issues_helper.rb:23:in `each'
    app/helpers/issues_helper.rb:23:in `issue_list'
    app/views/issues/_list.rhtml:16:in `_run_rhtml_app47views47issues47_list46rhtml_locals_issues_list_object_query'
    app/views/issues/_list.rhtml:1:in `_run_rhtml_app47views47issues47_list46rhtml_locals_issues_list_object_query'
    app/views/issues/index.rhtml:63:in `_run_rhtml_app47views47issues47index46rhtml'
    app/controllers/issues_controller.rb:91:in `index'
    app/controllers/issues_controller.rb:90:in `index'
    /usr/lib/ruby/1.8/webrick/httpserver.rb:104:in `service'
    /usr/lib/ruby/1.8/webrick/httpserver.rb:65:in `run'
    /usr/lib/ruby/1.8/webrick/server.rb:173:in `start_thread'
    /usr/lib/ruby/1.8/webrick/server.rb:162:in `start'
    /usr/lib/ruby/1.8/webrick/server.rb:162:in `start_thread'
    /usr/lib/ruby/1.8/webrick/server.rb:95:in `start'
    /usr/lib/ruby/1.8/webrick/server.rb:92:in `each'
    /usr/lib/ruby/1.8/webrick/server.rb:92:in `start'
    /usr/lib/ruby/1.8/webrick/server.rb:23:in `start'
    /usr/lib/ruby/1.8/webrick/server.rb:82:in `start'