SharePoint Online: Unable to create custom form – Could not save the list changes to the server

I am not able to create custom form for any list across the whole tenant and getting the error:

“Could not save the list changes to the server”.

I am working in SharePoint online and using SP Designer 2013. I am the site collection administrator.

It was working before and stopped working a few days back. I have tried following things but the issue is yet not resolved:

1) Checked “Allow custom scripts” from admin center.

2) Add and customize pages permission is granted.

3) Cleared designer cache.

4) Checked using another site collection administrator account.