Thank you for your post.
We eventually receive the crash reports from Apple, but the crash reports do not let us know what the customer was doing at the time of the crash. It would help us more the next time this occurs if you post the crash report up to and including the crashed Thread.
When the crash occurs opening another file, what can you tell me about that file being opened? What layout is it supposed to display? What objects are on the layout?
The sorted portal could very well be the issue, but I don't want to rule out all possible scenarios. If you temporarily unsort the portal that you think may be causing the crash, do the crashes subside?
Any other information you can provide may be helpful in narrowing down the possible cause(s).