unlocking Bootloader without erasing userdata?

Hello,
i have a Nexus 7 2013 edition.
I updated the device and something went really wrong.
I want to unlock the Bootloader to flash it, but it don't work, because everytime i try to unlock it, the tablet wants to erase userdata. This is were the Nexus stucks.

When i try to unlock the device, this happens:

Code:

---

C:\Program Files (x86)\Minimal ADB and Fastboot>fastboot oem unlock
...
(bootloader) Unlocking bootloader...
(bootloader) erasing userdata...

---

I tried to erase the partition without and with "-u" and "-w" but everything failed (i aborted because it went forever)

Code:

---

C:\Program Files (x86)\Minimal ADB and Fastboot>fastboot erase userdata
******** Did you mean to fastboot format this ext4 partition?
erasing 'userdata'...
FAILED (status read failed (Too many links))
finished. total time: 392.412s

C:\Program Files (x86)\Minimal ADB and Fastboot>fastboot erase userdata -u
******** Did you mean to fastboot format this ext4 partition?
erasing 'userdata'...
FAILED (status read failed (Too many links))
finished. total time: 187.879s


C:\Program Files (x86)\Minimal ADB and Fastboot>fastboot erase userdata -w
******** Did you mean to fastboot format this ext4 partition?
wiping userdata...
Creating filesystem with parameters:
    Size: 28521246720
    Block size: 4096
    Blocks per group: 32768
    Inodes per group: 8176
    Inode size: 256
    Journal blocks: 32768
    Label:
    Blocks: 6963195
    Block groups: 213
    Reserved block group size: 1024
Created filesystem with 11/1741488 inodes and 153337/6963195 blocks
target didn't report max-download-size
wiping cache...
Creating filesystem with parameters:
    Size: 587202560
    Block size: 4096
    Blocks per group: 32768
    Inodes per group: 7168
    Inode size: 256
    Journal blocks: 2240
    Label:
    Blocks: 143360
    Block groups: 5
    Reserved block group size: 39
Created filesystem with 11/35840 inodes and 4616/143360 blocks
erasing 'userdata'...
FAILED (status read failed (Too many links))
finished. total time: 203.907s

---

Can someone tell me how i can unlock the bootloader without erasing the userdata partition?
Or can i use another tool instead of adb+fastboot?
Can someone help me?