Now, I don't know if this is just my specific case, even if the Disk Utility was to blame, but I'd like to share what happened - because someone may find it useful. I already mentioned what happened in this thread. In short, I tried to create a Bootcamp partition with the Bootcamp assistant, it failed and told me to use Disk Utility to erase the bootcamp partition and try again. I tried to do this, but couldn't - I was left with a partition on the HDD part of my Fusion Drive that I just couldn't merge with the rest of the HDD. It would just give the "Operation Failed" message. Not knowing what else to do, I created an Yosemite Install USB and booted from it. I didn't revert to Yosemite, but used the Disk Utility there instead (which is the old one). I easily merged the two partitions without loosing any data. Rebooted back to El Capitan DP1 and everything is fine now. So, a warning - I don't know if anyone made a bootcamp partition from El Capitan, but the Disk Utility is possibly broken when it comes to repartitioning corevolume drives.