Posting the directions here because a few days ago when I tried to access the AVIC site it was not working. Not sure if posting links in here is forbidden but will give it a shot
This will repair the boot loop loading screen.
First off, you'll need a replacement SD card, the one I used was still stuck in the headunit as I used to use it for firmware updates and had a few tracks on. The image file is just over 8gig but I used a 64gb one because it was what I had. I'm sure a 16gb would work fine. This was formatted to ex-fat.
Here is a link for the IMG file download (credit to Jamesl75 who posted it on the other thread)
https://mega.nz/file/SdJCgKCJ#13zrFlogMCWNohCvuFsDFcC1uniHgufVZJxZlQPHUag
Once you've downloaded that file, unrar it and save it on your desktop.
Next download win32 disk image
https://sourceforge.net/projects/win32diskimager/
Open win32 diskimage, open the img file from your desktop then select the SD card drive... lets it do its thing for 4-5 minutes. Note the finished SD card was unreadable on my laptop... at first I thought this was a problem, but turns it it worked perfectly fine.
Next involves the stripping down of your headunit to get to the memory card. I used this youtube video below. Did everything except disconnect the last ribbon connecting the metal face backing to the unit... managed to angle the black plastic backing to give me just enough space to get the old card out and the new one in.
Then fit back into your car and it should work!! It did for me, and literally took me all of 20 minutes of get done and enjoy your working headunit again