thank you all for trying to help, but it seems I still don't have a solution. rknichols is right, it's 120 MB instead GB, but I'm sure I saw somewhere the 8th column of lsof is displayed in KB. Probably I'm wrong about it, so it's not the problem in deleted files either.

Right now I'm thinking the problem is that somehow system can't see all that extra GB's I added to the left. Confusion is great, and even without that extra GBs, there should be more space available...... I'm on the brink of formatting it and somehow restoring / partition, but I see there will be problems there ...

I had the same issue, and found the solution in a hidden directory.

Go to your home folder.

Press ctrl+H to show hidden files and folders.

Delete the folder named ".local".

And presto! I got all my drive space back!

I am running on a new, fresh installation that's less than 24 hours old when I had this issue and solved it. I download a lot so my free space is absolutely critical to me. I reinstalled because I failed to see how simple the solution was before, just like the OP.

The folder will return, but simply delete it again if it creates a problem again.

This has to be a bug in a recent update as I have never had this issue till recently, and only after updating. I don't update frequently to avoid such issues as long as possible, but it does happen from time to time and the solution to such problems is usually so simple most people over-think and miss it.

This information may come to late to be of use to the OP, but it will be useful to others who come along with the same issue.