hereby an overview of the issue:
1) upgraded Prestashop from 220.127.116.11 to 18.104.22.168. This shop is using a multistore feature (Shop1 & Shop2).
2) in Shop2 there are all world's countries / states enabled (using special characters)
3) after the Presta upgrade, One Page Checkout modules do not work in Shop2 (tested with 3 different OPC modules - same issue for all). In Shop1 OPC module works fine.
4) before the Presta upgrade there were no problems
5) according to a specialist the issue is caused by the character encoding error