Strange 'oe' character on plain console

[Grischa Stegemann <grix7-usenet@yahoo.de>]

Since I've installed Sarge I have the following strange effect on the plain text
console:

Whenever I use a program which draws an user interface by ascii line characters
(e.g. aptitude) instead of all the border lines this strange combined 'oe'
character is printed everywhere.

Now I have a 2.6.7 kernel running without any framebuffer stuff and the locales
are all configured to simply 'en_US'. The graphics adapter is a Matrox G550.

It's only on the console, not in any terminal within a X window.

Would be grateful for any hint,
Grischa