AltGr Key in aixterm and xterm

Hi all,
I’ve come across a problem that doesn’t seem to have a logical explanation.
Here’s the situation:
Host : AIX
Lang=de_CH (Swiss German)
Client: XP-Ger (same for Win2000 ENU, with Swiss German input locale)
Terminal emulator : keyboard settings to Swiss German

The problem:
When connecting to the host with xterm, the AltGr + [number key] work correctly.
ex. AltGr + 1 = ? , AltGr + 2 = @ , AltGr + 3 = # , etc.

However when connecting with aixterm, the AltGr key no longer works.

Any ideas?
Thank you!