Customize Function not working

I’ve seen multiple chains on this problem but none of the fixes have worked for me to date. Every time I try to open the customize option on my live website (maxibransdale.com) I just get the loading circle of doom.

I want to change my theme but don’t want to activate if I don’t have access to the customize option. I’ve tried a few of my previous themes and they all have the same problem on this site so I’ve no reason not to think the new one will be just as problematic so I really need this fixed.

Currently using Reddle and hoping to change to Sela.

Please any help or suggestions will be most welcome. I’m running out of ideas.