When I try to Expand a rollup, I get "This form is invalid"

Summary:

Content (please ensure you mask any confidential information):

When I try to expand (double click) on the Alternate Hierarchies roll up, I get the error "This form is invalid". What would cause this?

Version (include the version you are using, if applicable):

Code Snippet (add any code snippets that support your topic, if applicable):