Installation Fritzbox 7362SL failed red power flashing fast

I am a real newbie, starting step by step using several tutorials for the installation of OpenWRT 21.02 on a Fritzbox 7362SL.... :wink:
I made it to run the phyton script like this:

python openwrt-21.02.0-lantiq-xrx200-avm_fritz7362sl-initramfs-kernel.bin
> SETENV memsize 0x07aa5000
< 200 SETENV command successful
> SETENV kernel_args_tmp mtdram1=0x87aa5000,0x88000000
< 200 SETENV command successful
< 200 Media set to MEDIA_SDRAM

Is this how it sould look like?
Then finally the power LED is flashing red very fast.

According to what I read the box shoul after this be available under but this is not the case.
Thus I can't copy the sysupgrade file using SCP to finish the installation...
I did it several times (power off, running pyhton script again...), same effect.
Can anybody help me out what's wrong? What may have caused the fast red flashing power LED