So I just played through the polyamorous route real quick to make sure that it wasn't broken, and I was able to get the right ending, but I also didn't get any errors, so that's pretty weird that you were getting them. Luckily, that shouldn't affect your ability to get the poly ending.
There should be a scene shortly after the nightmare where Gunnar and Dyson show up to give you the option to date both of them, and that's where the "Both" comes in. If you're not getting that, then the problem is happening somewhere before that point.
After that, there should be a karaoke date with both boys, and then the kidnapping stuff. If you keep Gunnar alive during all of that, then you should be on the right track.
I really wish I could be more helpful!