Photoshop CC 2014.2.1/Camera Raw/Win 7 SP1 64bit
ACR Window is full screen mode -
Select adjustment brush, reset all values to default, use adjustment brush -
A modal error dialog is displayed BEHIND the ACR window. Even when acknowledged (get to it with windows means since ACR does not allow any control) it immediately comes back and PS application has to be stoppped with task manager.
The behavior with ACR window not in full screen mode seems normal and after the error dialog is acked the ACR window allows to make changes.