I record in X1d and mix in X3d. I've never had a session freeze (that wasn't plugin-related *StevenSlate is 90% the culprit*).
Maybe start troubleshooting from the start, even if it seems lame and wasted time :(
For example, create a blank project in X1d. Save it. Open it in X3d. That ok?
Now go back and create a project in X1d, record a single track. Open in X3d. That ok?
New project in X1d, record track, add some plugs you often use. Maybe 1-3. Open in X3d. That ok?
Continue till fail in that fashion.
If a blank project kills X3d, I'm not sure what to do.