"An unexpected error occurred. Please try again later." when adding new project to a monorepo

Hi,
We are using a paid subscription of SonarCloud and we use it with our Azure DevOps.

We follow the instruction here to convert a standard project to a monorepo project and add new projects to the monorepo project. We have 1 project added (linked from Azure DevOps’ repository).

Now, when we try to add new projects to this monorepo, we always encounter the error “An unexpected error occurred. Please try again later.”. I can confirm that Enable support for monorepo settings is enabled in the project.
Here is what we are doing: Analyze new project > Setup a monorepo > Choose the same repository > Add a new project with a different key (all displayed validators are green) > Save configuration.

I know this is very vague and I tried to look for more documentation on this but wasn’t very lucky. Any helps or leads would be very helpful. Thank you.