CQL Illegal State Error for 'null sortby personal.lastName personal.firstName'

In trying to launch the find-user plugin to assign a proxy (see UIPFU-8) I ran into this problem:

• Go to my development system at http://localhost:8080
• Click on the Users app
• Search for "d" (since we no longer get a list of users straight away)
• Click on diku_admin
• Click the pencil icon at top right.
• Open the Proxy/Sponsor accordion (which does not change the URL).
• Click "+New" in the "is proxy for" section
• Yields an error alert-box:

ERROR: in module @folio/users, operation GET on resource 'records' failed, saying: CQL Illegal State Error for 'null sortby personal.lastName personal.firstName': org.z3950.zing.cql.cql2pgjson.QueryValidationException: cql.serverChoice requested, but no serverChoiceIndexes defined.

This might be a consequence of recent changes to how the query-function for ui-users is configured.