I would like to inform you I have successfully fixed the issue.
Probably the problem was that the name of my user home directory contains space and special characters. So, the exact path which pgAdmin uses for log and reach the pgadmin4.db was the following:
c:\Users\Laczkóczki Ákos\AppData\Roaming\pgadmin4\
I think it wasn't able to reach the pgadmin4.db file because after trying to launch pgadmin the size of the db file was 0.
I have modified the config.py file as the following:
DATA_DIR = os.path.realpath('c:/development/tools/PostgreSQL/pgadmin/data')
else:
DATA_DIR = os.path.realpath(os.path.expanduser('~/.pgadmin/'))
So, I changed the DATA_DIR to a custom place which path doesn't contain any special thing and now I am able to launch pgAdmin and it seems it is working fine.
Thanks for your help!
Best regards,
Akos