When you update ispconfig, this command is executed: chown -R ispconfig:ispconfig /usr/local/ispconfig Problem is that this command changes the owner of exsiting bills: /usr/local/ispconfig/interface/invoices/* So you stop being able to modify them from billing module
You are right, the current ispconfig updater takes care to restore the directory permissions of the invoices directory but not of the existing invoices inside. I've added this to the bugtracker.
