After updating Magisk to v26.3 using the App (direct flash method), my phone only booted into the fastboot mode.
I fixed this by re-flashing the current Magisk (v26.3) using TWRP recovery. In order to do this, I needed to rename the Magisk APK to .zip (else TWRP 3.7.0 won’t recognize it). Then I flashed it, which only took a couple of seconds. After that, the phone booted normally again.