I set the partition manually,
I guess they rolled the dice, right?
Sorry, I don't understand what you mean.. Do you mean that setting partitions in Calamares is not recommended?
and so I just tried to shrink Window's main partition,
How?
I wonder how you can deliver such crap with such a long time in siduction. 
I am sorry... I would be glad to provide more info, but I thought it was clear. I tried using Calamares to set the partitions. I used it to shrink the Windows partition, created an 800MB fat32 partition for /boot/efi in the new empty space and used the rest (of the empty space) as ext4 for the root partition.
Everything looked good and started the installation, but as soon as it started, it gave the error message above.
On the other hand, I now have rebooted with the live ISO and did the same partition scheme with gparted. All went well and I was able to install siduction using the created partitions.
Indeed, I should have thought of that earlier, so I apologize for the noise. But is that a well know problem with the installer?
I was also surprised by how poorly it ran on Virtual Box, but I don't have to worry about that now, I guess...
Sorry, again, for the noise.
