In MCP 1.8 it seems to be a LWJGL issue. It returns for example for "ß" an event key value which is greater than 0.
See also:
I am really sorry if this is a duplicate 
How to reproduce (I think you have to use a German keyboard layout):
- Open the control settings
- Change a key for a action (example: Left) to for example "ä"
Affected keys:
- ^
- AltGr
- ß
- ´
- #
- +
- ä
- ü
- ö
Even some of the keys works later, for example AltGr doesn't:
It sets the used key to LeftControl, but later no matter if I press LeftControl or AltGr the action assigned to AltGr will be performed
Why this is only partwise maybe the US layout:
- Using Z or Y works correct, even they are switched on the US layout
- Pressing Strg (=Left Control) works, but AltGr gets also to LeftControl
Note:
- "Open Command" is default assigned to "Slash" however "Slash" is with the same layout "#" on the German keyboard. This means pressing "#" opens the chat and lets you enter a command