Have you tried restarting your PC, or does that still lead to the same problem?
Either way, could you please file a bug report? It would be very helpful to get a copy of your client's log after you close it for the first time after a re-install, and another copy from when it gets stuck at the "Preparing Assets" screen.
The log file is found at C:\users\\AppData\LocalLow\Wizards of the Coast\MTGA\output_log.txt. It gets overwritten each time you start the client, that's why I ask for a before/after copy. It would be useful to know any unique aspects you know of about the network your on. Are you going through a proxy? Are you playing behind a firewall? things like that.
It sounds like the client might not closing cleanly, and resources that it needs to get to are effectively locked so the next run can't get them.