I have a similar problem to this issue here. I have a Custom Connector, which has no authentication. The custom connecter calls a HTTP Request Flow which updates an item in SharePoint. The Custom Connector was created from a PostMan export. The Flow gets triggered successfully when called from PostMan and when testing from the Custom Connector itself.
It does not work when being called from PowerApps. I call the connector from a button OnSubmit formula, but get this error message :
The Custom Connector request itself has a Path and Query parameters :
Can someone help with finding the cause of this problem, as it's driving me nuts!