Holy cow. I just spent the last 45 minutes banging my head against the wall with this exact same issue, trying everything I came across. Nothing worked.
Turn on Layer 0...so painfully easy. Totally worked. Thank you!
as I described in another post, it has to do with what layer your materials are set on. As they are probably (and should be) on layer 0, if you turn that off (as happens when using layer isolate) you will loose the ability to osnap to lots of aec objects. as an experiement you can take the standard wall type, change the layers to the layer your walls go on and then isolate the wall and will still have all your snaps. I DO NOT REOMMEND this in practise but it will illistrate the bug that Adesk introduced into ADT/ACA and didn't resolve until version 10 or 11.
My work around working in ACA09 : I created a very simple lisp routine that will turn on layer 0 and regen and then I can keep working. I would like to redefine the Layerisolate routine to keep layer 0 on but I haven;t got that far yet!
(defun C:L0 () (command "-layer" "m" "0" "") (command "regen")) ;;Change to layer 0 and regen
I have 2011 and none of these things have worked yet. Though I am not sure what the "contextual ribbon toggle" is but I have switched the one in OSNAP settings, turned on and off layer 0 to no avail. Any help would be appreciated. This has happened to me twice before and I have no clue what resolved it, it just seems to have resolved itself or a good reboot did it. Not sure.