Actually, when cleaning files such as Dawnguard during modding, you come to learn that there are a lot of obsolete files that cause glitches and bugs in the DLCs. Some of them block specific routes and others are so random. To be honest, from my findings, it's just as buggy as the original game or Dragonborn but it's because Bethesda never spares the time to properly test their games before releasing them.