The current pre-release of the client ("pioneer" in the version) is only compatible to itself. So you can only play with other testers. Please be aware!
We need the full log of the debugger - please share that with us next time it happens.
We're on top of one particular issue that causes a lot of hard-crashes (90% of the reports so far are related to that one particular file), that will be hotfixed next Monday.
