If you change the query festuca%20a to either festuca%20 or festuca%20ab it works.
life.catalogue.es.EsException: [1:434] [bool] failed to parse field [filter]. Caused by: [1:434] [bool] failed to parse field [must]
at life.catalogue.es.EsUtil.executeRequest(EsUtil.java:373)
at life.catalogue.es.nu.NameUsageQueryService.executeSearchRequest(NameUsageQueryService.java:91)
at life.catalogue.es.nu.search.NameUsageSearchServiceEs.search(NameUsageSearchServiceEs.java:62)
Found some of these in the error logs: http://api.checklistbank.org/dataset/3LR/nameusage/search?content=SCIENTIFIC_NAME&sortBy=NAME&fuzzy=true&type=prefix&limit=20&q=festuca%20a
If you change the query
festuca%20a
to eitherfestuca%20
orfestuca%20ab
it works.