Some players have reported the MTG Arena client not progressing past the Preparing Assets screen. One possible workaround for that is to go to https://support.microsoft.com/en-us/help/2977003/the-latest-supported-visual-c-downloads and to download and run vc_redist.x86.exe. Here is a direct download link: https://aka.ms/vs/15/release/vc_redist.x86.exe.
Another workaround that's worked fairly consistently for players is using a VPN to connect from another country like the U.S.
We are investigating a fix to the root cause.