The error is not caused by your widget implementation. It happens because the app is running in React Migration modeand still relies on legacy Dojo layout behavior, where Placeholders are restricted to layouts only. After switching the project to React-only, the Placeholder works as expected and the error disappears.
If the issue is resolved, could you please close the post? This way, others who run into the same problem can immediately see the solution.
A Mendix Placeholder can only be used in a Layout to define where page content goes. It is not a normal widget, so it cannot be placed inside another widget’s slot that allows child widgets. When you try to put a Placeholder into a pluggable widget slot, Studio Pro checks the rules and throws the C6700 error. You could try keeping the Placeholder in the Layout and placing your pluggable widget inside that Placeholder instead. If you need a reusable structure with editable content, use Layouts + Placeholders, and treat your pluggable widget as a normal widget inside the layout, not as something that contains a Placeholder itself.
We just found out, that this issue is only appearing when working in an Mendix 9 Project, that was upgraded to Mendix 10 and using the react client in Migration mode and there for stil using older Dojo component.
After switching to react only everything works.