Hi, thanks for your answer.
Yes, caching is enabled. (Tried it with deactivating as well, does not help either).
I guess this is the wrong forum to ask/post this anyway. I tried to implement the code into a static simple index.html site and it opens a new window as well. So to me it seems the client made some wrong settings when creating the buy button. (Unfortunately I don’t get access to this shopify account to check back).
Just put this in there as someone could have used this before and had some similiar issues.