Steps to replicate the issue:

• Mark a new page as translatable
  • (example)
• Go to https://meta.wikimedia.org/wiki/Special:AggregateGroups in order to add it to the existing group
  • (example belongs in the "Abstract Wikipedia" aggregate-group)

What happens?:

• The page-title doesn't autocomplete as it should/normally does
• Error message in popup, "invalid group"

What should have happened instead?:

• The page gets added to the group