This happens every time I open the app, and reinstalling does not work.

I am having the same problem. I did a uninstall. Removed all the asset data ( that is surprisingly not being removed when you do an uninstall ) and installed it again. As soon as the menu was shown I closed the app in the proper way, restarted it again it spend 2 minutes checking assets because my last game session apparently resulted in an asset error. It must be noted that the app also still remembered my login credential so apparently still not everything was removed with the uninstall. I have tried 3 games in total. All of them resulted in a crash or disconnect, with no changes to get back into the game on time because of 2 minutes assets checking.

Every time I start up the Mac version of Arena this process (screenshot) begins taking up to 30 minutes. I didn't experience any issues during the last session so I wonder, what's happening with this routine.

Also happening frequently to me too on Mac.

Upon launching the game, I am given a screen that says "An asset error occurred during your last play session. Validating asset files to ensure stability, this may take a while.

This happens at every launch, even when the client is closed through Adjust Options > Exit Game

Since the last update (2021) MTGA crashes when i start a game (even against sparky)

Arena won't Install, and I've tried all the advanced troubleshooting with no luck. It says the Windows Installer Service could not be accessed.
Have tried the following:
- Restart PC
- Restart router
- Disable Firewall
- Trying another admin account
- DNS Flushed
- Edited hosts and reset it
- Re-installed Windows 10 to a fresh copy
- Downloaded new installer file from: https://magic.wizards.com/en/mtgarena
- Tried Installer from: https://forums.mtgarena.com/forums/threads/58489

Still nothing working and I get the same error:
"The Windows Installer Service could not be accessed. This can occur if you are running Windows in safe mode, or if the Windows Installer is not correctly installed. Contact your support personell for assistance."

all chechks are good... but can't install

Which OS? Are you giving admin permission? Is your firewall ok with it? Are your drivers up to date?

Check these questions and it should work ;).

P.s. just replying cuz devs seem to read yet not reply on low vote posts.
check official forums for fast community replies ^^

Good luck!