We have just migrated our properties from DTM. When I try to view an AA: Set Variables action for any of our page or event rules, I get a blank page with "There was an error loading this extension view".
Possibly related, when I try to open the Global Variables accordion in the Analytics Extension configuration, nothing opens. I just get the down arrow but nothing appears.
I moved a whole bunch of custom configuration code out of the “custom library code” (and set it to "manage the library for me") and into the “configure tracker using custom code” section, and as if by magic the global variables and Set Attributes parts of the tool are working again.
So it must have been something in that code which was breaking it.