If you are using the IDE locally, you must mark the branch as a Tool in Settings. This is because only tools can access external websites.
Control Panel -> select website -> select branch -> Settings -> mark as Tool.
Inconvenience only. If you are the owner of the IDE website, eg. when you downloaded it for local development, you will get an error when deleting a file.
The file is actually deleted. The reason is that the Boomla tools are loaded 2x if you are the owner, thus the delete operation is attempted 2x, and the second fails. This is an inconvenience only, doesn’t affect functionality.