Update 2: Update: it seems the decryption issues are getting resolved as many players are now entering the game!
UPDATE: Amazon Games have now officially responded to the launch issues, stating that they are working with Valve and clarified what the issues are. You can read the full statements here.

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Lost Ark has finally arrived to the West and Founder's Pack players are able to play it right now! In case you can't see the play option, restart your Steam client, but you may also run into a connection timeout as everyone is trying to download the latest update (even with the preload done there seems to be a 400 MB addition for some players). There's no workaround for the issue yet, but we'll keep you updated.

The game alternates between trying to download the content and timing out, however it seems to be a decrypting issue on Steam's side:

The free to play version arrives in 3 days and here are some of the key information you will need relating to the launch:

• Which Server to Choose and Avoid in Lost Ark? Community Spreadsheet and Streamer Servers

• Lost Ark Launch Release Official Patch Notes

• Lost Ark Launch and Director's Trailer, Pre-Load Available Now

• Critical Lost Ark Launch Information and Official Posts: Servers, Queues, Item Shop, Content and More

• Lost Ark Server Language Information

• Lost Ark Content at Launch: Limited Tier 3, Many Continents, More Improvements and FAQ

• Lost Ark Founder's Packs Contents and Video Showcase

• Lost Ark Twitch Drops, Prime Rewards, Giveaways, Ark Week and More

Source: News Latest Topics https://www.icy-veins.com/forums/topic/64082-lost-ark-head-start-has-launched-steam-connection-issues-and-all-important-information/