Our office has four 5775's and all of them have the same problem (and nobody at Xerox tech support seems to be able to help us other than shrugging and saying "I dunno, it shouldn't do that").
Sometimes a user will try to Copy or Send a fax (anything that requires them to press the green Start button) and as soon as they hit the start button the screen saying 'Please respond to options in the window" and refuses to do anything. There are no options in the window to respond to, and performing a quick restart on the machine seems to fix it, but within hours the problem will come back up. This has resulted in us constantly restarting our machines all day.
That is really strange. The first step I recommend is do a firmware upgrade. The last version available in the internet is this one. 061.131.221.10401.
A xerox tech has actually come in and upgraded our firmware to a version apparently even higher than that (we're on 061.132.222.03800) and it still didn't fix the issue (it was a lot more prevalent before, happening almost constantly but it's still happening now more often than we'd like).
In that case, the second step is do a full reset. Do you know if the Xerox tech did a upgrade from previous version or did a clean install by ALT BOOT? If he did an upgrade you should call him again and ask to load as ALT BOOT or do a NVM reset. Do not use any clone after. Simply configure your device from scratch.