Getting past step 2 in checkout while registering a new account

Hello - first time poster so sorry if this is in the wrong place.
We have a website https://classynails.co.uk/ where on the checkout - if you try to register an account in step 1 - everything works fine and then when you have finished filling everything in for step 2, you click continue, the little loading wheel briefly appears before disappearing and nothing else happens.
We are running Version 1.5.6.
This only happens when trying to continue while registering an account. If we do the same process as 'checkout as guest' everything works fine. I have tried disabling the T&Cs confirmation box incase that had anything to do with it but I get the same results. So it is the actual continue button.

The site is using an evolved version of the lexus maxxshop template.

Any help/advise/points in the right direction would be great because I have reached the end of my technical abilities (which were limited to begin with) in trying to figure this one out.

Thanks in advance

Using a custom theme. Switch back to the default theme and see if this issue can be replicated. In addition, due to the countless bug fixes integrated on more recent versions, I would suggest upgrading to at least v2.3.0.2 release of Opencart.