Well, it's kind of unusual and serious problem. For two or three weeks circa 20 GB from my home folder just vanished. It's not a matter of any big files, even hidden, because I checked it out many times and I used disc analyzer.
How does it look from my perspective? Maybe (but it is only guess, not proof) it's a fault of VirtualBox. At least, the problem began after installing this program. I choose an option of create virtual disc or something like that, but then I claimed there is no need to do this, and soon turn on the program. However, the trouble occur. The computer started to work hard and there was shown info, that there is only 900 MB free space (more or less, I do not remember, but it should be 20 GB).
What I remember is then I wanted to uninstall VitrualBox and X86 virtualization solution, but it was just impossible. I tried to switch on root, but after log out there was shown a column of communiques and the only way to turn off the computer was power cut. When I log in root, some problems still existed, I didn't see any possible large files or something, and what is more there was a communique-info about any error of programmer.
But nevermind. Finally I uninstalled both VirtualBox and X86 virtualization solution. However the trouble still existed. I also deleted some large programs, like PlayOnLinux, so it allowed me to get a few GB free space. Still too less, but...
It is quite bizarre. The free space in my home folder changes regardless if I download or delete files. Well, there was a period, when I didn't download or delete, just shut down or turn on my computer, and free space rised. After delete Playonlinux, there was 3,5 GB free space, and after next day it was circa 8 GB, then even over 10. But it is still too little.
Two days ago, I had to copy some files form my pendrive. They were only powerpoint presentations, so the size of them was about few MB. After plug in pendrive and copy files, the computer slow down again, and the communique about no free space on disc occur again. It was necessary to me to work on it, so I deleted two large files (about 6,5 GB) to work calm, but after next turning on computer, this free space also vanished!
I do not know if I make myself clear. It does not look like there is any large file on my disc, but if something still shortened the disc size. I do not know if it is any tip, but when I want to edit a file made in LibreOffice, I cannot do this, because there is communique about use to read only or open copy. Before this accident it had never occur.
And now, there are results from terminal commands (my logs are embed here): http://ubuntu.pl/forum/viewtopic.php?f=127&t=171771
but no one can help me.