After some time Xournal just didn't start.
I got a tip, that this is the software-s problem, and I should use another handwriting-notepad. Next step is to find one.
For Xournal I had to reinstall ubuntu, hoping Xournal starts, because I saved some important notes in its format.
NEWS: Actually, I reinstalled Ubuntu, and installed the newest Xournal (0.4.2.1) according to the instructions on the xournal homepage. I hope it will work. From now on I will save everything to pdf :-)