That cleared up the problem with that error. Then I get a timezone warning (I get that one all the time, telling me I can't rely n the system timezone but must set it manually). I fix that error by manually removing from the code the parts where it's trying to insert a date/timestamp into the "orderdate" field of the orders table (summary insert) (to be honest, I don't understand why php needs to insert a timestamp, instead of letting mysql insert the time automatically).
After that, the order goes through, but no new records are entered into the orderdetails table (which is called "rosters" in my DB).