I'm using slightly different software versions, but this just works:
mkdir /dev/shm/bitcoin; bitcoin-qt -datadir=/dev/shm/bitcoin
It saves on /dev/shm (which is more or less a RAM-drive in Ubuntu). Can you test this, just to see if that gives you the same error?
the only way to make the bitcoin-core to work is to let download the block chain to the default directory but that's not what I want.
Just in case: In your screenshot, you ticked pruning. That means it'll fit on the default location too.