I just recently starting getting an error when I try to save the map in ArcGIS Online new map viewer:
What type of layers are you using in the map? Is it a new map you are creating in the Map Viewer or updating an existing on? If you can, can you share the layers you are using to create the map?
I was having the same "numThumbs" problem.
Triple check that all your map layers are not time-enabled. I just checked all (20) layers in my map and found one that was time-enabled. I turned that one off and everything saves perfectly.