hi jjk,
hey thank you very much for your advices! I just did as you suggested me; I even uninstalled the OPC from joomlaprox.com. The funny part is, that as soon as I also disable the native incl. the OPC Ajax. I simply receive an order confirmation without payment procedure. What I mean is; As soo as I hit the "pay now" button, I receive the order confirmation, incl. the error message above (screenshot). I also kept the pre-loader off for the onlineshop. Anyway, hopefully we're kinda getting closer? (Btw, I kept the site online now and I also re-activated the OPC from joomlaprox.com)
Thank you again, jjk for your great help and for sticking with me! In case I won't make it to reply you today, I wish you a happy new year!
best regards!