Allow character to step on physic-objects

This is hopefully a simple question:

Back when I started my project, my character was able to just stand on objects I simulated physics on. However, when I do this now, the object and the character just start shaking uncontrolled, and at some point the object will fall through the ground. I’m not sure what I changed. I looked through my Character and set the capsule-component’s collision-settings back to default, nothing changed though. I also tried changing “Can Character Step Up On” and the walkable slope angles, but this doesn’t seem to make any difference. How do I reenable this? Also: What can I do to improve the quality of the physics apart from activating CCD on the actors (This helped a lot fixing objects falling through the ground, but it still happens sometimes unfortunately)?