Hi Reiner,
I can't reproduce what you are saying. I did this:
1. Updated to the latest build.
2. Open an v3.5 project, which has groups.
3. Expand some nodes and select one of them, then hit the "LEFT cursor" key. The focus is properly moved up to the parent node.
Are there any special circumstances under what this happens?
Does it happens all the time? (old projects, new ones, etc)?
I know this could sound silly, but I will ask you anyway : Is your LEFT cursor key working? It may be that it is sticked or dirty, so when you hit it, it is like if you hold it for ever.