Solved
Custom sim skin doesn't work, my sim is invisible
So I had a self-made Costum Sim that looks like Herman Monster. Somehow the immigration from the old game data to new Game folder didn't go as planned and he's almost invisible. You only can se shade...
You have to be careful when making these. You can damage your install folder if you end up overwriting files. Here's a discussion about a similar issue. This glitch that wasn't introduced in Legacy Collection: https://www.reddit.com/r/thesims1/s/7hCcr8skYU
