Dug in a little deeper, it looks like the problem is in acs-subsite/www/admin/group-types/one.tcl. It sets up return urls like so:
set return_url_enc [ad_urlencode [ad_conn url]?[ad_conn query]]
....
set add_group_url [export_vars -url -base "../parties/new" {{party_type $group_type} {add_with_rel_type composition_rel} {return_url $return_url_enc}}]
so the value gets encoded twice. I think it should be just
set return_url [ad_conn url]?[ad_conn query]
....
set add_group_url [export_vars -url -base "../parties/new" {{party_type $group_type} {add_with_rel_type composition_rel} return_url}]
