Welcome Katharina.
There was a bug when upgrading from 7.0 to 7.1 but I thought I had fixed that.
Can you make sure you have the very latest 7.1.0.11 (Help > Check for updates)?
If you have got the latest version and it's still a problem, can you:
1. Attach a copy of your current preferences (ini) file to this thread so I can determine the problem
2. Open the filter visibility settings (View > Task Attribute Visibility) and make an arbitrary change and click OK. I believe this will fix things.