I'm also having this issue. Have looked through the directory there doesn't actually appear to be any logs produced. The game itself doesn't actually crash it just freezes which is probably why it isnt logged.
Having tested it, it appears to happen under these conditions: Portalgun is held, upon trying to enter a portal, upon being near a turret bot and when interacting with the entities from the portal mod (companion cube and co.)
For clarification this occurs with the Ultimate pack, Mindcrack appears to be fine.