I have the same exact problem with Screen Sharing and the gray linen screen. In my case, I have a MBP with two accounts (mine and my wife's) and an iMac with only one account. The only way that I can access my MBP account in the background from my iMac (if my wife's account is the active foreground account on the MBP) is if my MBP account has already been logged into. If I try to sign into my MBP account from my iMac I only get the gray linen screen.
Did you ever get a resolution to this issue?
Well actually I'm a little luckier than you, it's sort of a 1/2 chance for me. A lot of times when I try to control my account on the imac while my brother use it I get either a black or linen screen, but half of the times it will log in for me. But I haven't really try to observe the exact probability for that happening so I'll say it's 1/2 chance for now. And no I don't think I've ever got a solution to this. Oh and a small tip, if you can't log into your account on the MBP in background and that it shows up as either the black or linen screen, when you finally get to your MBP DO NOT log into your account with the account switcher on the menu bar from your wife's account, the top layer of graphic will freeze and you would have to use the log out shortcut to logout your wife's account. Best to first return ot the login screen first than log into your account that almost always works.
I'm pretty convinced this simply a GUI issue. Here's why. Today I tried logging into my account on my MBP from my iMac (my wife's MBP account was already logged into but mine wasn't). As described above, the grey linen screen popped up and there were no icons in it at all. However, what I did this time was click where my login icon should've been, typed my password, and voila, Screen Sharing worked as expected. Taking this a step further, I tried again, but this time when the gray linen screen popped up, I clicked where my wife's login icon should've been, typed her password, and her screen popped up on my iMac via Screen Sharing. If I click where the Guest login icon should be, I get a message stating that the system will restart to a Safari-only guest user account (since I haven't configured a guest account).
Anyway, it seems that not all of the login screen elements are being transmitted over the Screen Sharing protocol properly; instead, all we get is the gray linen background. If you act and click as though the screen elements are there, Screen Sharing works as advertised.