Many AVM devices can be unbricked by a AVM recovery tool which is specific to your device.
Example: For Fritz!Box 7530 http://ftp.avm.de/fritzbox/fritzbox-7530/other/recover/
Search the corresponding recovery tool for your specific device on the AVM FTP server.
If you have a branded (e.g. 1&1) AVM device and you get the following message
The device contains basic settings adapted for your Internet Service Provider
when trying to go back to OEM firmware via the AVM recovery tool, you have to unbrand your device first. See the procedure below how to do this.
ftp -n 192.168.178.1
quote USER adam2 quote PASS adam2 quote SETENV firmware_version avm quote UNSETENV provider quit
Not sure if it's necessary to change the firmware_version variable.