Ok, here'S the thing;
At the very first I got rid of the SuperCache Extension. Right after I uninstalled the Helix framework, went to the jomshaper site, downloaded & installed the newest Version. And now this is the Situation: At the english front end site, the error message of the parameter Merchant e-mail is gone. I can proceed the order 'till I got re-directed. At this point I'm now getting an error message about the missing shipping adress (Screenshot), even I've already the shipment as you can see at the screenshot. However, at the German site of the onlineshop I'm facing a kind of different issue; As soon as I entered the order page, I were receiving the same message as were we started with the forum ( vmError: The parameter Merchant e-mail is required for the payment ( 1) [Screenshot]). But just now, only few minutes later, when I enter the German section, than the message just disappeared, and I can at least hit the payment button. And now, I'm getting the thank you message for ordering (Screenshot). Even I have a good feeling, that we're closer now, I think my site is kinda spooky

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Thank you very much for motivating me to find a solution and to stick with VM

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Very best regards!!!