I think it would be necessary to try another install. Since the 14 (exactly on 2016-01-21) I uploaded fixes to partman-prep which might well improve on the situation here.
The jist of it, at least, is that there already is a PReP partition being created on your system (the boot partition listed in your comment), except the flags might not be set properly on the device.
I think it would be necessary to try another install. Since the 14 (exactly on 2016-01-21) I uploaded fixes to partman-prep which might well improve on the situation here.
The jist of it, at least, is that there already is a PReP partition being created on your system (the boot partition listed in your comment), except the flags might not be set properly on the device.