Question: I have a USB live key created with Linux Live USB creator. I used all the defaults, and got a standard version of Ubuntu. When I try to boot into it, I get the error above. I went through the rest of the threads with no avail.

Things I tried:

* Renaming the files (my files were already syslinux.cfg, etc.)
* Typing in `mboot.c32 -c boot.cfg` into the boot:\_\_ prompt

Any help would be appreciated!

Answer: 
It seems for everyone it has a different solution, for me, the solution was to format my Flash drive with the FAT filesystem and make a boot drive with the Universal USB installer and voila, worked like a charm.
I know this is super late but just in case anyone else has the same problem. I had this same issue on my USB flash drive. Well I figured out it was because I had deleted everything from my flash drive and just moved the ISO and the installer back to the drive. 
(I only deleted it all because I had tried to install two different OS on the same USB and it was messing up my Ubuntu and I couldnt figure out what wasn't Ubuntu so I deleted everything.) 

So the problem was that I had just highlighted and moved those two files back on the usb stick and it didn't go back to working right until I went back in and ran the installer like before (I used the Universal USB one) and it will go through the whole renaming the disk and everything. Then when it finished I noticed all of those files I had deleted. After that it was all back to normal. 

Also I re-downloaded both Ubuntu and the installer just in case (but Im sure it was nothing wrong with the original downloads now). 

So if you get that error, first think back if you accidentally deleted any of your Ubuntu files from your flash drive. This is probably not the case for the original poster but for anyone who happens across this thread as I did who has tried all of those options, maybe this will help you.