element is already the child of another element domaincontext

The best way to troubleshoot this is by stripping of elements from your xaml on by one.

In my case, I had a ChildWindow which hosted a domain context. I don’t know why by this is sufficient to trigger the error.

       <!-- Commenting out the offending component was sufficient -->
       <!-- web:SamDomainContext x:Key="SamDomainContext" / -->