Thank you, that worked.
The detail page now displays. There is just one more problem. And I am not sure this even caused by the switch to MySQLi.
On the detail page the customer has the the choice of three options before putting the product in the cart. Also, depending on the choice, the item number, the price and a pzn-number (that's a special German pharma number) change instantly, without page reload.
After the switch to MySQLi the first option only shows on choice instead of three. And the item number, price and pzn-number don't show at all. Because of this problem (as a fail safe), the put in cart button also is not there.