Closing this one out for those playing from home - the issue seems to be caused by having the UI Password on FPP enabled.
There were a bunch of errors in the logs about incorrect credentials, despite xLights not prompting for it on an upload output. Testing via FPP Connect (which does prompt for password) - brought up 5 password dialogs, and still didn't work.
Disabling the UI password enabled the outputs to be uploaded successfully.
There were a bunch of errors in the logs about incorrect credentials, despite xLights not prompting for it on an upload output. Testing via FPP Connect (which does prompt for password) - brought up 5 password dialogs, and still didn't work.
Disabling the UI password enabled the outputs to be uploaded successfully.