More info: https://www.reddit.com/r/SS13/comments/ ... continues/
The best way to deal with this issue is to right-click the Dream Seeker title bar and select "Options & Messages" to view download progress.
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DO NOT disconnect until the download is finished, otherwise you'll need to restart the download process. Once the download completes you should then be able to move freely.
If the download completes and you're still having issues, reconnect.
If you reconnect and you're still having issues, wait for a new round. When a new round starts, something in the client resets to allow players to join freely and easily when they have already downloaded the needed resources. This is also relevant if you wipe your Byond cache folder; You'll likely need to wait for a new round to start before you rejoin as your client will crash continuously until it does.