I had the exact same problem yesterday when flashin a benq. I tried to make the bootables using Iprep 8.6 and 8.8 but it was hit and miss. I used the iPreps from xbins though. I never thought that downloading the iPrep 8.8 from this site would make a difference. I thought that whatever is available on xbins is essentially the same stuff as what's available over the forums.
Bottom line, I had the same problem using the iprep 8.6 and 8.8 from xbins. After several Ipreps reinstalls I managed to load up all 3 firmwares, but something went wrong with the dumping stage, so I had to abort the session. Since this particular 360 was for a buddy who did not want to do xbox live I gave up on using the 8 versions so I flashed it using iprep 6.2 and ixtreme 1.4 and worked flawlessly. My buddy is happy. I'm happy. 360is happy!
I was just trying to make sure nobody has addressed my questions in another thread before I start my own.
Thanx....this site rocks!!