I had a similar problem when I changed resolutions to a lower number. What happened, is the buff window reset to it's default size, but kept it's location where I had it at the higher reso. The result was it was sitting off the viewable area of my screen.
I had to manually edit my ui_character_server.ini file and change the position of the buff window. I just slammed it about 100px from the top/left and saved. When I logged back in, the buff window was sitting in the middle of the screen, and I was able to stick it where I wanted to.
Might help, might not. Worth a try though.