With the big updates of Plasma 5.7.4, KDE Framework 5.25.0, and KDE Applications 16.08 Kontact and Kmail crash at startup.

I have 3 machines running Tumbleweed and only my home Desktop computer has this problem. The home Desktop has an Nvidia GT-640 graphics card, and is running the nouveau driver. My laptop computer has an Intel graphics card, and my 3rd Tumbleweed computer is a KVM virtual machine. These do not have this issue.

Googling, I found this Arch forum post.

Starting Kontact as indicated in post #8 allows Kontact to work as it did before.
I also found this KDE bug.

I don't know if this is a KDE bug or a nouveau bug.

I hope this helps someone else.