Did you water after repotting? And was the pot the same size/type?

If you have any photos, that might be helpful to narrow down the issue.

Honestly, I didn't take any special care when repotting my Mesembs.

Only I am particularly careful watering them after repotting. If I suspect I have given too much water, I dry out the pot with a warm heat source/fan/paper towels to suck the extra moisture away.

Better to wait and water very lightly after repotting (days, no rush) until all the fine roots re-establish themselves. Otherwise the plant may dehydrate with no fine roots, despite sitting in saturated soil.

Then if it's watered more and the roots finally grow in, the plant is massively over watered and will turn to mush.

Not sure if that helps at all.

No, I did not water immediately, and I did not trim the roots either. That was some time ago, and the latest incident was noticed after I repotted, so I have no pictures unfortunately.

The potting soil is mostly sandy, so it crumbled all by itself. When repotting with bare roots, I place a cone-shaped pile of soil inside the new pot, at the desired height. I then place the plant over that pile and add more sand to the desired level. I do my best not to disturb the roots too much.