Problems installing linux mint on clean hard drive: Server authorization directory error

I've had some issues with a dual install of windows and ubuntu lately. I decided to clean things up by wiping my hd and just reinstalling linux mint.

I've made a bootable usb. When I select the option to launch linux mint, which would start a live version previously, during the launching i get the following error:

'Server Authorization directory (daemon/servAuthDir) is set to /var/lib/mdm but is not owned by user 111 and group 121. Please correct the ownership or MDM configuration and restart MDM.'

When i OK i get the message: (shown in a linux mint-environment) The computer has only 0 bytes disk space remaining. When i 'ignore' this one, nothing happens anymore. Everything just stops.

I have been stuggeling with problems on this computer for a while now. I hope someone can help me to clean things up and make a whole new install.

EDIT: now I think of it, there are some more clues which may be helpfull. When booting using PartedMagic (only way i can still boot now), the disk health application shows my drive is 250GB in size, which is correct, while the gparted tool can only find 232GB. I've been trying to find and answer to this problem, to no succes, and it may be linked.

Roel