Openbox Syntax Error on boot

Dear Mr.Szymon,

Please check for us this trouble and how to resolve it.
Thank you

Something probably went wrong when you first started the desktop environment (maybe a power cut). You can remove this file and it should regenerate when you log in after reboot:

rm ~/.config/openbox/lubuntu-rc.xml

we follow your instruct and clear this trouble.
Thank you very much.