I can see many issues related with this topic.
How can I debug it?
Hello, welcome to the forums.
There is a problem with opening or accessing the database file for Kanboard… I would look into the database settings and make sure that it can be accessed.
Your error logs may show hints on what to check other than that
yeah, but my data/db.sqlite and its parent has 777 permissions 
I mean this morning was working, all this time ( 3 months ) but today die 
in that case I didn’t know and you were not very clear with your situation. I presumed as you were a first time poster that the installation is new also. Clearly not.
Your problem is related to the database connection. It is someething to do with the server or the files in your installation.
Sorry I cannot help any further. Maybe KanboardSupport plugin will help if you are able to install it.