Hello,
To be sure to understand well the situation, when you switch to Protostar it solve your issue, right ?
"2. Changing the template did make the trick and it did work but how can I solve this issue?"
I'm afraid that the root issue is from your template, maybe a distant library that change or is now deprecated ?... Can't be sure of it, but it seems that you have to contact your
template creator to get clues and we hope solve your issue.
One thing you can try is to open your browser javascript console before clicking on the finish button. You might get an error message caused by a javascript issue on the page which might explain why it doesn't work and that error message should provide more insight on what's going on and why.
Regards