So I think I figured out why this weird stuff happened.
For my storyline, I found it too easy to just let her stay at the fire station because she just always had a place to sleep, shower and eat. So I thought about getting rid of the fire station. However, for my story I needed a reason for it. So I used the DebugEnabler mod to make a fire there (how ironic, I know :D ), to let it burn down so she needed to get to an other place. However, I've totally forgotten that I have a mod called "more intense fire" or something like that, so the fire would spread faster and would be...well, more intense. It took me too long for it to go out, so some sims from the neighborhood came over and tried to put it out. When some of them catched fire, I thought, "oh well, maybe I should just reset the lot with MC." I did exactly that, and this was the moment her inventory and relationship panel resetted completely. It's still weird though, considering she wasn't on the lot when I resetted it.
Oh, and @PlumDiddy, sims suddenly teleport to another location not far away is a thing that is quiet normal to happen in my game no matter which save I play, but it never had any negative impacts on gameplay, so I live with it :D It's not like they teleport to the other side of the town, only like one meter away, so it's not as bad as some might think ;)
