How to insert "[" character

Directly after starting IntelliJ I can use the characters [ and ].

On a German keyboard it is created using "Alt Gr" + 8, "Alt Gr" + 9.

After a short time this is not possible anymore.

Instead of inserting the character it jumps to the previous { character or  } in case I want to type ].

This problem is IntelliJ specific, at the same time the characters can be used in any other application. 

After restart of IntelliJ it works again for a while.

Is there any other solution that a restart? How can I prevent this from happening/why does it happen. It is really annoying. I now even switched to Eclipse - which sucks, but at least the editor works.