I have a group which has Fastpath queue's JID added as a member (firstname.lastname@example.org). It shows in the list with red asterisk. 4.3.0 sometimes throws exception when opening this group or trying to add more such members (or say email@example.com). Although it actually adds a member after all. In contrast 4.2.3 hangs in loading the page when you try to add such members or open this page, but refresh fixes the issue.
Simple reproduction recipe;
Go to a group edit page
In the "Members of this group" section, in the "Add user:" text box add a user along the lines of "firstname.lastname@example.org" where not-my-domain.com is not the local XMPP domain, and not a domain that is connected via a server-to-server connection.
Note; that group edit page then becomes inaccessible (with the above stack trace) until /something/ (not sure what) expires.