Hi there
This looks like an issue with the admin access key configured on the co-GM’s side. The config might’ve been lost if they’ve switched browsers or cleared cache/cookies/localstorage at some point, but it should be an easy fix.
Please make sure that they’ve got an access key configured or the folder shared with them (if your old access key was timed it might’ve also expired, so generating a new one / re-sharing is the best bet), and ask your co-GM to check that they’ve got access to the shared assets library as a source in their FilePicker
as explained here
Hope that helps you out!