How can I prune my Bitcoin Core correctly? The other way did not work for me. Have I done any unnoticed mistake?
From your description it sounds to me that you may have edited your configuration file while Bitcoin Core was already running.

prune=1100 is a startup parameter. It will take effect once you restart the program.


If you add a parameter to the startup instructions,  parameters start with a dash (-prune=1100), e.g.:

"C:\Users\&lt;username&gt;\AppData\Roaming\Bitcoin\bitcoin-qt.exe" -prune=1100


Please make sure you use the correct path for the actual directory you installed Bitcoin to.
If you add it to the configuration, the parameter must be listed without a dash in the bitcoin.conf:

prune=1100


If you're unsure how to locate your Bitcoin data directory, please see: Where is the configuration file of Bitcoin-Qt kept?


There is no need to manually delete any files from your Bitcoin directory, or to start over the initial blockchain download to enable pruning.
You merely need to restart Bitcoin Core after you've edited the bitcoin.conf. If you don't know how to restart a program, you may find it simpler to shut down and restart your computer.