Hello @mickster58,
Thanks for writing in!
Pro builder is not loading because there is a JS error on the page. The error is caused by your custom JS script. You have added this in the header custom JS section:
<script type="text/javascript" src="https://assets.inplayer.com/paywall/latest/paywall.min.js"></script>
Please be advised that this is not a valid JS code. You are not suppose to insert JS file in here because the resulting output will be wrap with <script> tag.
If you need to insert this line in your header, please install “Insert Headers and Footers” plugin. Once installed, go to Settings > Insert Headers and Footers > Header and insert that code.
Please let us know how it goes.