Seems that bug found...
We have FB auth button on the personal data form step screen.
Today our local operator temporary block Facebook.
After my customer told that can not pass his data into the form. Both buttons NEXT and BACK stop working on this step.
After short research we saw that facebook api is not reachable (errorcode 500) and bookly.js also has fault.
We delete FB login support in the settings and after plugin works fine.
Seems the solution is to check by JS on a customer side if FB api works as well otherwise do not use FB login feature.