Did you test it on different browsers? Does the error persist across all the browsers? Does clearing the cache help?
Check if rebooting the machine resolves the problem.
Are you able to add the layer to the webmap directly?
Yes I've tried it on more than one computer, and tried it having cleared browser cache.
As for your final question, sorry could you give a bit more detail about that? I'm quite new to AGOL.
Thanks for your help.