I found the solution for my problem! When i looked in the file "recovery.log", i saw that the problem is a file in the directory /data/extm/. Then i saw, that there is only one file in this directory, which is exactly 3GB in size. Then i remembered, that the new Xiaomi.EU ROM 18.104.22.168 has a new option "memory expansion" (i don't know, if this is the correct word in english. In german the name ist "Speichererweiterung"). And this memory expansion is 3GB in size. I disabled this option in the rom, made a reboot and the file disappeared. Then i tried the TWRP-Backup again and all was working fine!