Stuck in Fastboot mode with "No valid slot to boot" after Magisk rooting attempt

For a Pixel 3a XL with an unlocked bootloader, I tried to flash a boot.img file patched by Magisk following [this guide](https://wccftech.com/how-to/root-google-pixel-3a-xl-using-magisk-tutorial/) , using [bonito-sp1a.210812.015](https://dl.google.com/dl/android/aosp/bonito-sp1a.210812.015-factory-872afcc7.zip) from [Google Factory images](https://developers.google.com/android/images) . After I successfully completed the whole process from "Step 2: Rooting Google Pixel 3a XL" section and rebooted, my Pixel became stuck booting with a "G" logo (and progress bar animating). I tried to fix it with these commands, which resulted in always booting in Fastboot Mode, error message "No valid slot to boot". ./fastboot flash boot_a "boot.img" ./fastboot flash boot_b "boot.img" ---- **How can I make my Pixel boot normally again *without loosing any of my user data* living on it?** **Alternatively, is there a way to back up all my user files from Pixel with eg. adb, given the broken booting?** ---- I'm able to boot in recovery mode if it can be useful.