I started the first mission "Helping Hands" now twice. In both attempts the same thing happened:

Before i can commence my final assault on the enemy home system the game hands out error reports about the "Marauder fleet" which i can ignore to continue. Shortly thereafter the game starts lagging and soon freezes to a complete halt.

Thank you for your reply.
I prefer not to make another account and it seems someone else reported the very same bug just today.