Power BI APP access For External users - Invalid Content Provider

Hi Fabric Community,

I have an issue sharing a Power BI app to external users.

We are using audiences to control access to the reports in the app.

The external users are listed in our Azure AD.

The error message they get is 

We Couldn't install [App Name]

ServerError_ContentProvider_InvalidContentProvider.

I've checked their access to the APP in the Audience and there are reports visible to them.

Any Ideas where to check next?

Thanks in advance.

DKHughes55