This is a long shot, but you could try using a different version of the bootloader. Here's a link to the one found on the
Cell Addon CD. Click on the link, then right-click and save the
otheros.bld file. Put that on some sort of media (USB flash, compact flash, SD, MS, CD, etc) in this exact location:
/PS3/otheros/otheros.bld
Then insert the media while in the game OS and use the Install OtherOS function again (same as you did with the YDL 6 DVD). This installs to the PS3's flash memory and overwrites the bootloader you've already installed. Then boot to OtherOS and try the install again.
If this doesn't work and you want to go back to the YDL bootloader, just follow the procedure again with the YDL 6 DVD.
Since I haven't seen any reports of this I'm guessing it could possibly be an issue with your YDL 6 install DVD, but again I really don't have an idea of why it's doing that.