Ok, I put the iso on another usb drive, ran the install again. This time I made sure I formatted the /boot partition. The first time I didn't check the format option for that partition and right after it said it needed to format it and I clicked ok so I assumed it formatted it. This time I didn't get the error message upon installation and the system booted fine.
Unfortunately, I lost the grub entrys for my previous LM13 installation, but I guess I won't need it if this installation works properly.
Btw, the removable drive applet doesn't work (its icon doesn't appear) appart from that, no noticeable bug. Display settings seem more stable.
Custom build w/ Gigabyte Z87MX-D3h | Intel Core i5 4670 | 8GB DD3 | 250GB SSD