Caliper brake -- middle piece loose
Some brake calipers have a small locking screw with which you can adjust the rotation of the brake caliper without having too loosen the bolt with which the caliper is mounted to the frame. Check if your caliper has such a locking screw and if tightening it helps. It should be approx 2mm in diameter.

The afore mentioned locking screw can be seen here: https://www.parktool.com/assets/img/repairhelp/dual27.jpg

Image source: https://www.parktool.com/blog/repair-help/dual-pivot-brake-service