Schedule Apex Interface not found error

Of course, the class still has the Schedulable Interface (otherwise it wouldn't even show up in the lookup window above):

global class ApexMonthlyJobs implements Schedulable
{
  /**
    Execute is called when the scheduled time for this class arrives.
    All Classes which have to be executed daily can be initialized and run here
   */
  global void execute(SchedulableContext sc)
  {
    System.debug(LoggingLevel.ERROR, 'ApexMonthlyJobs started');
    // Only execute at the start of every year
    if ( system.today().month() == 1 || Test.isRunningTest() )
    {
      PersonelReminder pr = new PersonelReminder(PersonelReminder.ReminderType.JUBILEE_DATE);
      pr.remind();
      PricebookCloner pc = new PricebookCloner();
      pc.clonePB();
    }
  }
}

Is there anything I can do to fix that?