How can I save a Google Pixel 4a stuck during boot?

I got a Google Pixel 4a that is stuck during boot without obvious reason (maybe it did an automatic update and restart during the night). Whenever I start the phone, it only shows the "Google lettering" but won't continue to boot. I can access fastboot (vol- + power button). That way, I have also access to the phone via cmd console on my computer. Still, when I try to access recovery mode, it will also get stuck when it shows the Google lettering. Also, a wipe or factory reset is not possible via power button + vol+ as I cannot access recovery mode. Resetting the phone via https://flash.android.com is also not possible with the error message: > Device Unlock Disabled > > For security reasons, you need to enable "OEM unlocking" in order to flash your device. See the device preparation instructions for more information. So what other options do I have to get the phone back running?