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I've cloned and I've tested your form. I am able to replicate the issue you are having. When I clicked Next Botton, Your form was stopped working. I've check your source code. This issue is related with Form Tabs Widget.
Can you change the arrangement of your widget as you see below?
At that time, It will work more properly.
Here is how it works:
Hope this will help.
If you have any question or issues, please do not hesitate contacting us. We will be glad to assist you.
Considering that you last question is not related to subject of this thread, I have moved it to separate thread and we will provide you an answer there shortly.