I am getting the SetNormalTexture error frequently too and agree the guide has become almost unusable.  Example click tick box
    next to quest in Guides panel and it often throws this error, especially if the panel should resize. I have repaired and reinstalled losing
    all my saved variables. I have tested it with no other add ons and still
    continually getting this error. Installed 3.027 but it was happening
    with previous versions. I had hoped 3.026 would sort it. Obviously not.

    It might be a specific guide, can you tell me which guide you are using take a screenshot

    can you test with a brand new character