From what I've gathered, the installer needs to install some package to create the encrypted partitions. It may be that it fails because it's trying to install to the partition that's going to be encrypted, although I don't know that for sure.
One solution might be to unpack the iso myself, pre install the needed package, repack & burn the iso.
Any advice on how to do that, or whether there's another easier fix available?
The computer is a new dell inspiron 5577, with 512gb ssd, no spinning disk.