Strange Syntax error after succesful checkout

[JoeSchiaffi]

Hi there,

 

I'm on the final tests of my new shop, but today I find out that after a succesfull checkout I got a strange browser pop up error that say:

 

parsererror<br>SyntaxError: Unexpected end of JSON input

 

Don't know what it is, the order is placed correctly, I receive email confirmation and all, but what is this message? And HOw can I make it not appear?

Thanks

[JoeSchiaffi]

Trying to checkout with different browser (firefox) I've got the same error, but a little more complete explanation. Where do I have to look to fix this problem?

Edited July 28, 2017 by JoeSchiaffi (see edit history)