Hi Owen
I have come across this as well
Create group with update capability
Invite other member (creator/ publisher to group)
Owner creates a new story map and manually shares to group with update capability
Other member signs in to ArcGIS Online and opens story map in the group.
Clicks configure makes a change and then Publish
This error shows up.
I couldn't find anything in the network console that shows why we are getting this error.
However when you refresh the storymap the changes appear to have been made.