The reason seems to be related to multithreading and the shadercache. A hotfix is already planned.
I found that the game starts more reliably when your PC is somewhat busy in the background while starting the game (so the chance is less that several threads are able to run "at the same time" and get forced to run one after another). Nevertheless, you will have to be patient and retry starting until it works - until a patched client gets released in the updater.