I came across this issue and my resolution was similar, thought I'd share to help anyone else in the future
If you published your data with a definition query on it (in my case from a file GDB from ArcGIS Pro 2.2.4), and your edit template for the hosted feature service does not satisfy that definition query, you cannot create new features in that feature service. Updating the template to satisfy the definition query will resolve the issue. This is an odd one, since that definition query no longer has an bearing on the data (you can then go ahead and edit the new feature to fall outside of that definition query).
Also, if you've configured a layer on your hosted feature service to enforce an attribute as unique, and your template does not provide a value for that attribute, a new feature cannot be created. Similarly, if you attempt to add a new feature that's duplicating that unique attribute, the feature will fail to create (no warnings provided).