Thanks a lot. I don't think the QHSUSB works here. This only fixes the bootloader as far as I could see. But the bootloader is fine.
I also tried the SD-EXT MOD (I think you mean this one
http://forum.xda-developers.com/htc-sensation/development/mod-sd-ext-boot-mod-cm12-t3068867). But with all mods except the first one, TWRP crashes while "Installing kernel". The first MOD runs through, but at the end, when it says "Updating partitions", that it is still unable to mount data.
To send you the phone and to send it back costs around 30€ and I don't think you will work for free... I'm not sure if this is worth it... Do you think there is a chance you can fix it?
As far as my understanding goes, there are corrupt blocks on the data partition, created by a bug in the emmc-controller, which was triggered by TWRP. Are those blocks really defect now, or does only the filesystem think they are defect?