I added the workaround I just found to the UESP.net wiki

If you choose to accept Harkon’s gift, you may die when he bites you and be forced to go back to your last save. (Workaround: Before talking to Lord Harkon, set the quest stage to 180. Follow Harkon and talk to him; he will say that you have forsaken his gift, but will offer it to you

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