Is a crack in the basement of an old building harmless?

Hello,
we bought an old building from 1972 seven years ago.
The crack was already there when we bought it and is located on the wall to the neighboring semi-detached house in the basement. I never gave it much thought. Should it be checked or is it not serious?

Set a plaster mark and observe. If the plaster mark breaks, something is moving. Otherwise, no concern.

Set a plaster mark and observe. If the plaster mark breaks, something is moving. Otherwise, no cause for concern.

Hello,
I applied some hobby plaster over one of the slightly wider spots in November. The plaster looks the same today as it did two months ago.
Now I wonder: did I use too little plaster? Or should I have noticed a change if the crack in the plaster was moving and can now be reassured?

Your marking looks good.
In the first pictures, however, it looks as if someone had already filled something along the crack? Or am I imagining that?

Your marking looks good. But in the first pictures, it looks like someone might have already applied putty along the crack? Or am I imagining that?

Yes, that's true. We bought the house in 2014 and I think the cracks haven't really changed. The house is already 50 years old. Does it look more like a harmless crack or could there be something serious behind it?