I also had that problem and could work around by loading nls_ascii.ko
module manually. It turned out that the vfat mount to access the EFI
partition failed due to lack of ASCII IO charset.
So I guess the reason for the bug is the lack of a loaded nls_ascii
module at the time when the partition should be prepared and mounted
during the install.
Regards, Thomas Wuensche
EMS Dr. Thomas Wuensche e.K.
HRA Neuburg a.d. Donau, HR-Nr. 70.106
Fax : +49-8441-81860