When will NXT web client officialy close? And should I withdraw all my NXT and NXT assets into the local desktop client? Otherweise I would lose them?
The myNXT web wallet (third party service, do not mistake for the official Nxt Client) is closing as announced in December 31.
You have two options, either transfer the contents of your myNXT wallet to an account you created with the Nxt Client and that you have the passphrase for, or you download a myNXT wallet backup (recommended) so you get the passphrase for the Nxt account you have in the myNXT wallet.
You have all the info in the myNXT wallet itself,
https://wallet.mynxt.info, and in the myNXT FAQ page,
https://wallet.mynxt.info/faq.phpIn any case you only have this month to do it, so don't leave it for later.