I was able to fix this issue using the hint in ubuntu bug report: just replace gtk-auto-mnemonics = 1 by gtk-auto-mnemonics = 0 in the theme you use.
I use Fedora theme so it was in Clearlooks (/usr/share/themes/Clearlooks/gtk-2.0/gtkrc)...
Of course, auto-mnemonics a new feature of gnome is now disabled, so this is only a quick fix not a real one!
I was able to fix this issue using the hint in ubuntu bug report: auto-mnemonics = 1 auto-mnemonics = 0
just replace
gtk-
by
gtk-
in the theme you use.
I use Fedora theme so it was in Clearlooks (/usr/share/ themes/ Clearlooks/ gtk-2.0/ gtkrc). ..
Of course, auto-mnemonics a new feature of gnome is now disabled, so this is only a quick fix not a real one!