First: cannot access front end? I dont understand your problem to acces to the front end. The front end is not closed you can go and work and if you want to try the editor in the front end, when you clic in your "Front end editor" button, you only need to use the credentias that i shared with you. I just cheked and it works perfectly
Second: obbiously you dont find any problem because the problem comes in the front end editor, not in back end.
I am trying in other sites where i am working and the problem persist. All of them are sites with a new installation of joomla and components, without any modification
If you see de video demostration, the problem coes when i try to duplicate an addon, but not always, like you see in the video. For example, when i import one of your templates and i diplicate one of his addons, it duplicates correctly but when i add a new "text block" addon and try to duplicate, hole pagebuilder desapears. This is just a random exaple, i have this problem in other situations when i try to duplicate.
After some tests, the problem also comes when i try to change block column number and/or dispositions (4+4+4+12). I can make a video example if you want