The search function for selecting recipients uses JavaScript instead of a backend call that the other searches use in the Participant Center. The issue is most likely related to the contacts in the participant's address book. One of those contacts might have a name that conflicts with the JavaScript's ability to search. A contact must have an email address and may or may not have a first/last name. If they do not have a first or last name, then it should say <no name>. If the name field is completely blank, then that might be what is causing the issue. Recreating the contact will fix the issue.

It is currently unknown how a contact may end up in this state, but if an address book was imported from an external system, the issue could be that the names attempted to import with non ASCII characters, or some other conflict that our system is not compatible with.