I have a single instance where a member of the public is trying to access a web map and getting this error. This person is completely outside my organization. I have been unable to replicate it, despite testing the map across multiple browsers on multiple computers and my phone. I recommended refreshing the map and clearing their cache, which they said they did and are still having problems. The layer in question is a hosted feature layer on ArcGIS Online and is publicly shared. I am suspecting a firewall or other security issue thanks to this blog, but that doesn't help me help them.