I had a similar problem after upgrading - what I found was that I had a local version of Network Atlas installed on my polling engine. I upgraded the version of Atlas on my laptop and was modifying maps there, and I got the error.
Once I went back to the polling engine and installed the upgraded version of Atlas on the server, my maps started showing up normally.
Hope that helps!
we have this issue tracked in our system. If you are accessing the maps through the "All Maps" page, wait for 30 seconds after the All Maps is loaded, and then click on a map. Eventually, just wait for 30 seconds after the issue occurs and then refresh the page.