I have never seen this error before. Have you tried a different browser? If you can continue to reproduce this error. I would recommend opening up a trouble ticket with Support by clicking on Help > Report a Problem.
Brian, can you tell me exactly how you were able to fix this error? I just had it start showing up last night.
Can anybody help out here?
If you're using a self-hosted canvas instance, the new editor it is trying to load there is a now actually a completely separate web site, connected via LTI. The good news is you can run it on the same server, but you have to install and configure things, and it is NOT easy to get going.