I am also having this problem, which started last Friday. When we created our services, we checked the box under mapping capabilities that says "Allow per request modification of layer order and symbology". We did this so that our users can change symbology in their web maps and remove layers from the web service that they do not need in their map. When I first load the map service into our AGOL map, it works fine and I can configure the popup. As soon as I either change the symbology (even just the color) or remove on of the feature services, I can no longer configure the popup. Our ArcGIS server is 10.2.2. Also, I can reproduce this error from a blank map as well as an existing map, and it happens with every map service I tried. As a side note, we have access to a Portal that is 10.6.1, and I tried the same process and got no errors. Any ideas?
Thanks,
Linda
Here is a screen shot of the error I get from the console as soon as I make the change.