1. Go to https://translatewiki.net/wiki/Special:AggregateGroups
2. (Create a group and) add a translatable page to a group
3. Immediately click the icon to remove it

Expected result: page is removed. Actual result: error "The group parameter must be set" is shown and nothing happens; you have to reload the special page and proceed to disaggregate the page.

Note: can't be reproduced on WMF wikis because bug 42430 prevents the aggregated page to appear before refresh.
I consider it low priority because it's annoying but one rarely needs to disaggregate a page immediately after aggregating it.

Version: unspecified
Severity: normal