I had this issue with one host not booting off the virtual media ISO file. I tried a number of ISOs thinking it might have something to do with the files themselves but none worked. Looking at the iLO logs, I saw some strange timestamps:
First, the date’s are totally out of whack. Then I took a look at the system time in the system’s BIOS and sure enough:
Fixed the time up and now the machine boots off the mounted virtual media. So yeah, pretty basic – fix the system’s time.