I think I can consider this issue the strangest problem that I ever had encountered. I am unable to enter @ or € (or any key that requires the alt-gr switch on German keyboard) in iceweasel, evolution, pidgin, gucharmap but am having no troubles in all other applications I tried (kword, OpenOffice.org, abiword, gvim, urxvt) in an current squeeze system.
If anyone has any idea or hint what might be the cause of this I would be more than grateful!