The specified file does not contain a Fritz!os released for this device by AVM

Hello and welcome aboard

Greater/Equal than 6.50 = Only signed firmwares by AVM possible
Workaround: Recover it to lower than 6.50 first

Link: ( belgium version/branding: avme )
ftp://ftp.avm.de/fritz.box/fritzbox.7490/x_misc/belgium/

Link: ( german version/branding: avm/1und1 )
ftp://ftp.avm.de/fritz.box/fritzbox.7490/x_misc/deutsch/alt/

Easier: Install the new version using the bootloader. This will work even with future versions (as far as we know) and there's no need to reload the configuration (which results in a new SSL key/certificate and DECT devices have to be registered again normally) as it's necessary after a recovery session.

The new firmware may be loaded into RAM and after starting it from there, it will write itself into the active partitions ... you should switch to the alternative set prior to loading the image into RAM.

My GitHub-Repo contains some scripts to assist in this task - starting with "image2ram" to build the file for upload from a complete Freetz image and last but not least there's "eva_to_memory" to load and start the resulting file.

If the updated firmware < 6.50 and EVA/ADAM2 environment variable linux_fs_start 1 exists then change it to: 0
...and after reboot you have the older firmware back ready to flash a unsigned one.

This workaround was discovered by: PeterPawn

Ah...
I just bumped into this yesterday evening.
I wanted to upgrade a 7490 to Freetz after I updated it to Fritz!OS 6.52 a month ago.

AVM is for sure aware of Freetz and it is strange they are doing this.
Are they forgetting that most of their software is not written by them???

@PeterPawn Do you have a link to your work?

@frater:
Sure ... I have a link and even an account with the needed RSA key to access GitHub via SSH.

You did realize, that I wrote about "my GitHub-Repo" (funny mixup of German and English, as I can see now), didn't you? Could you guess, how you may find this repository?

The general approach by AVM to block unsigned updates since 06.5x versions was long overdue. It was too easy for average users to harm themselves.

I don't like to say, the devices are now really "hardened" against unauthorized modifications, but one more possible vector to attack them is blocked now.

The whole task (the "ultimate goal") needs much more such steps ... but even the longest journey begins with the first one.