To confirm maina man statement, I had the exact same problem with my Sharp XV-Z21000, I could not reset the lamp hours, and I have the same problem with Sharp XV-Z10000, I own them both. I tried 2-3 times to reset the lamp hours on the 21000 mounted on the ceiling, with exact the same way, holding down the enter, down arrow, right arrow, and then press on, with no luck.
Yesterday I tried again, but this time I had the projector uninstalled from the ceiling, and standing on his feet, because I will move it from my living room to my bedroom. The first time I tried the way sfm suggested, holding down the enter button and then press on, with no luck. I turned off the projector, let it cool down, and tried again after a few hours, but this time I tried the way maina man suggested, holding enter, right arrow, down arrow, and then press on, only once, not holding it down. Finally, success!