add migration that fixes the issue from #2591, re #2591, fixes #2622

Closes #2622

Merge request !1769

db/migrations/5.1.46_fix_for_biest2591.php

+<?php
+final class FixForBiest2591 extends Migration
+{
+    protected function up()
+    {
+        $query = "DELETE `tools_activated`
+                  FROM `tools_activated`
+                  JOIN `seminare` ON `range_id` = `Seminar_id`
+                  WHERE `seminare`.`status` IN (
+                    SELECT `id`
+                    FROM `sem_classes`
+                    WHERE `studygroup_mode` = 1
+                  ) AND `plugin_id` IN (
+                    SELECT `pluginid`
+                    FROM `plugins`
+                    WHERE `pluginclassname` = 'CoreParticipants'
+                  )";
+        DBManager::get()->exec($query);
+    }
+}

lib/classes/SemClass.class.php

@@ -44,7 +44,6 @@ class SemClass implements ArrayAccess
     static protected $studygroup_forbidden_modules = [
         'CoreAdmin',
         'CoreParticipants',
-        'CoreSchedule'
     ];
 
     static protected $sem_classes = null;