I guess that the ispconfig update might have reset the permissions of the folder where the invoices are stored. Please execute these commands as root on the shell to correct that:
chmod 770 /usr/local/ispconfig/interface/invoices
chown ispconfig:ispconfig /usr/local/ispconfig/interface/invoices
I have another question concerning the usage of the reminders emails. Is it possible with the billing module to send scheduled reminders to the customers (30 days before, 15 days before, 5 daye before)
There is no option for scheduled reminders available yet. I've added this to our feature request list.