Hi @Luise , welcome to the forum 
You may have not realised but the new sidebar is an important update that seems to be showing up in many Asana accounts this week! This has been announced a while back and I have since created a topic on this to help teams get prepared, which you may want to share with your team.
Regarding the issue you are experiencing, you can read up on the new ‘Projects’ section and revised ‘Teams’ section:
Since this is expected behaviour and not a bug, I am moving your post into the tips section.
And unfortunately, I do not believe you will be able to opt out of this.
Hope the above helps you and your team to adapt to this change!