You will need to add /rest in between, so for instance, “https://localhost:8080/rest/ApiName”
I just tried to do it that way, I tried: https://localhost:44364/rest/Administration/GetAssets and https://localhost:44364/Administration/rest/GetAssets
But it’s still giving me the same error 560:
Do you have an error message in your Studio Pro console when you try to access the REST service from a browser?
Can you try to delete the actionButton3 on your form and replace it with a new action Button?
I had this issue two times in my last projects and couldn’t figure out what the ‘generic’ error description is about. Replacing the existing button with a new button helped somehow. Let me know if it helped in your case too :)
Otherwise: Did you already try to access the local API via Postman first so you can be sure that the issue is Mendix related?