I had the problem of twisting gennacker halyard too.
It was caused by a grommit where the line was pulled trough.

BTW we use now a simple dyneema 6mm line without core for the halyard. at the point where the spinlock gribs, we inserted a core. The dyneema line is quite slippery and dosn't burn holes in to your spinny. On the downside it is hard to hold, but with the rubber gardening gloves we use it is perfect.