Not a good support from the plugin developer. They emailed me to create a ticket on their support forum. I did that and after that we have long communication and below is their final response
I have checked the issue in your end. Well to be honest at this moment. I
cannot say what might have caused the error there can be a loads of things.
You need to understand that in your site you are using a good number of
plugins. Also you have multiple block builders. The theme is also something
that you need to look into. Making the troubleshooting test I can realise
that there is a high chance of a plugin/theme conflict in the site. As you
are not a pro user I cannot proceed further and do more debugging to
resolve the matter in hand. I can only suggest you to try the qubely table
of content in another site you will find it working fine.
I have tried deactivating all plugins and using default 2023 them but plugin still has the issue.
According to them if we use paid plugin than only they will provide support.
