I am now having this same problem in ArcGIS Online. It started suddenly 3 days ago. I had just added a layer I created in ArcMap 10.5. I also had a major internet outage just after I added that layer. I can't open my existing content because it says the basemap is unavailable. Then, it says it is displaying the default basemap instead, but nothing displays. If I try to open a new map, no basemap displays. If I try to add any of the default basemaps, I get this.
So, I click OK, and then I try to add a basemap, any basemap, from the Basemap button. With every basemap option, I get an error saying Basemap <name> is not available. If I try to add any other layer (Add > Search for Layers >any random content in ArcGIS Online or Living Atlas) from esri, it says the layer is unavailable. If I add a layer from someone other than Esri, I can add it for ~50-75% of the time.
This is ONLY happening in Chrome. I have tried clearing the cache. I even uninstalled and reinstalled Chrome. I tried the CORS extension. Since it is clearly not a problem with our account, I have not even considered making any modifications to ArcGIS Online (I am an administrator but do not have call authority so esri tech support can't talk to me). Since I am able to do everything with no problems in Microsoft Edge at home as well as on my laptop in Chrome and on the school computer I use for teaching class, this is clearly limited to just Chrome on my desktop. Unfortunately, my desktop is my prime work station for all lesson planning, and I heavily use ArcGIS Online in my classes. I am very hamstrung at this point.
I would surely appreciate it if anyone has any ideas. Thank you!