I have reached out to Shopify Plus Support and they confirmed the bug with no quick-fix so far.
It has been confirmed that this is an ongoing issue within the theme editor. Our developers are already aware about this issue and are actively working on resolving it as soon as possible.
Currently, there is no suggested workaround provided to access the theme editor while the issue is still ongoing. I tried to duplicate the theme to see if you can work on it for now, but I still encounter the same error message.
In the meantime, we will put this ticket on hold while it’s still under investigation and I will send you a follow up email once we have new updates from the team. Please keep an eye on your email and feel free to reach out if you have further concerns.