I just installed my new server and I am getting the error message "Field 'client_salutatory_email_message' doesn't have a default value" when I try to create a new reseller. Once I commented sql-mode="STRICT_ALL_TABLES" in /etc/my.cnf the error went away. I will try to find more incorrect SQL inserts and post them here.
Look into database: table isp_isp_reseller have some TEXT filed mark as NOT NULL but don't have default value for it. When ISPconfig add a record. It don't have value to those filed. I don't know what is the default value. So mark it as NULL (uncheck NOT NULL propertie) of those record will resolve error.
