When I want to change and save the user permission of any menu entry I get the internal server error 500 and the following PHP error: Class 'Users' not found

This also crashes the whole Menu entries configuration page with the same error

It can be reverted by setting the relevant user_id in surveymenu_entries table to NULL.

Here's the whole stack trace:
#0

• /var/www/survey/framework/db/ar/CActiveFinder.php(182): CActiveRecord::model("Users")
  #1
• /var/www/survey/framework/db/ar/CActiveFinder.php(231): CActiveFinder->getModel("Users")
  #2
• /var/www/survey/framework/db/ar/CActiveFinder.php(48): CActiveFinder->buildJoinTree(CJoinElement, "user")
  #3
• /var/www/survey/framework/db/ar/CActiveRecord.php(915): CActiveFinder->__construct(SurveymenuEntries, "user")
  #4
• /var/www/survey/framework/db/ar/CActiveRecord.php(282): CActiveRecord->getActiveFinder("user")
  #5
• /var/www/survey/framework/db/ar/CActiveRecord.php(143): CActiveRecord->getRelated("user")
  #6
  /var/www/survey/framework/base/CComponent.php(614) : eval()'d code(1): CActiveRecord->__get("user")
  #7
• /var/www/survey/framework/base/CComponent.php(614): eval()
  #8
• /var/www/survey/framework/zii/widgets/grid/CDataColumn.php(142): CComponent->evaluateExpression("$data->user_id ? $data->user->full_name : "<i class='fa fa-minus...", array("data" => SurveymenuEntries, "row" => 3))
  #9
• /var/www/survey/framework/zii/widgets/grid/CGridColumn.php(231): CDataColumn->getDataCellContent(3)
  #10
• /var/www/survey/framework/zii/widgets/grid/CGridColumn.php(154): CGridColumn->renderDataCellContent(3, SurveymenuEntries)
  #11
• /var/www/survey/framework/zii/widgets/grid/CGridView.php(639): CGridColumn->renderDataCell(3)
  #12
• /var/www/survey/framework/zii/widgets/grid/CGridView.php(623): CGridView->renderDataCell(TbDataColumn, 3)
  #13
• /var/www/survey/framework/zii/widgets/grid/CGridView.php(579): CGridView->renderTableRow(3)
  #14
• /var/www/survey/framework/zii/widgets/grid/CGridView.php(489): CGridView->renderTableBody()
  #15
• /var/www/survey/framework/zii/widgets/CBaseListView.php(181): CGridView->renderItems()
  #16
  unknown(0): CBaseListView->renderSection(array("{items}", "items"))
  #17
• /var/www/survey/framework/zii/widgets/CBaseListView.php(164): preg_replace_callback("/{(\w+)}/", array(TbGridView, "renderSection"), "{items} <div id='tokenListPager'><div class="col-sm-4" id="massi...")
  #18
• /var/www/survey/framework/zii/widgets/CBaseListView.php(149): CBaseListView->renderContent()
  #19
• /var/www/survey/framework/web/CBaseController.php(182): CBaseListView->run()
  #20
• /var/www/survey/application/views/admin/surveymenu_entries/index.php(67): CBaseController->widget("bootstrap.widgets.TbGridView", array("dataProvider" => CActiveDataProvider, "id" => "surveymenu-entries-grid", "columns" => array(array("name" => "id", "value" => "'<input type="checkbox" name="id[]" class="action_selectthisentr...", "type" => "raw", "filter" => false), array("name" => "buttons", "type" => "raw", "filter" => false), array("name" => "title", "type" => "raw"), array("name" => "name"), ...), "filter" => SurveymenuEntries, ...))
  #21
• /var/www/survey/framework/web/CBaseController.php(126): require("/var/www/survey/application/views/admin/surveymenu_entries/index...")
  #22
• /var/www/survey/framework/web/CBaseController.php(95): CBaseController->renderInternal("/var/www/survey/application/views/admin/surveymenu_entries/index...", array("model" => SurveymenuEntries, "user" => 1), true)
  #23
• /var/www/survey/framework/web/CController.php(872): CBaseController->renderFile("/var/www/survey/application/views/admin/surveymenu_entries/index...", array("model" => SurveymenuEntries, "user" => 1), true)
  #24
• /var/www/survey/application/core/Survey_Common_Action.php(267): CController->renderPartial("/admin/surveymenu_entries/index", array("model" => SurveymenuEntries, "user" => 1), true)
  #25
• /var/www/survey/application/core/Survey_Common_Action.php(338): Survey_Common_Action->renderCentralContents(null, array("surveymenu_entries/index"), array("model" => SurveymenuEntries, "user" => 1))
  #26
• /var/www/survey/application/controllers/admin/SurveymenuEntryController.php(36): Survey_Common_Action->_renderWrappedTemplate(null, array("surveymenu_entries/index"), array("model" => SurveymenuEntries, "user" => 1))
  #27
  unknown(0): SurveymenuEntryController->view()
  #28
• /var/www/survey/framework/web/actions/CAction.php(109): ReflectionMethod->invokeArgs(SurveymenuEntryController, array())
  #29
• /var/www/survey/application/core/Survey_Common_Action.php(83): CAction->runWithParamsInternal(SurveymenuEntryController, ReflectionMethod, array("sa" => "view"))
  #30
• /var/www/survey/framework/web/CController.php(308): Survey_Common_Action->runWithParams(array("sa" => "view"))
  #31
• /var/www/survey/framework/web/CController.php(286): CController->runAction(SurveymenuEntryController)
  #32
• /var/www/survey/framework/web/CController.php(265): CController->runActionWithFilters(SurveymenuEntryController, array())
  #33
• /var/www/survey/application/controllers/AdminController.php(158): CController->run("menuentries")
  #34
• /var/www/survey/framework/web/CWebApplication.php(282): AdminController->run("menuentries")
  #35
• /var/www/survey/framework/web/CWebApplication.php(141): CWebApplication->runController("admin/menuentries/sa/view")
  #36
• /var/www/survey/framework/base/CApplication.php(185): CWebApplication->processRequest()
  #37
• /var/www/survey/index.php(194): CApplication->run()