Solution found: by checking md5 hash of install_piratebox.zip and of the openwrt image I discovered that those two files were incoherent. Actually, recently I downloaded install_piratebox.zip but *not at the same time*
openwrt-ar71xx-generic-tl-mr3020-v1-squashfs-factory.bin ...
Now everything fine, thanks also to good conversation in the IRC.
openwrt-ar71xx-generic-tl-mr3020-v1-squashfs-factory.bin ...
Now everything fine, thanks also to good conversation in the IRC.