No protections, but I experimented a little more:
I have tried different sizes of tar files via WLAN and via LAN: One with 45 MB, the other with 55 MB.
The smaller works, the larger not. This is true for two installations on different sd-cards (8 GB & 16 GB). WLAN or LAN makes no difference.
I also made the larger file smaller by removing a large file (e.g. artwork.db): Now it works!
So for now I will simply remove the folder „cache“ from the tar-file. I have to rebuild it anyway with a new installation of squeezeserver.