The Able Bindery Z39.50 client requests records in element-set "F", and those requests are being rejected with BIB-1 diagnostic code 5 (Specified element set name not valid for specified database). That is straight-up wrong: the Z39.50 standard (section 3.6.2, "3.6.2 Comp-spec Omitted") states:
A target must always recognize the character string "F" as an element set name to mean "full"; when it is applied to an abstract database record, it results in the same abstract database record (i.e. a null transformation).
A target must always recognize the character string "B" as an element set name to mean "brief" record. This standard does not define the meaning of "brief." Unless the origin knows the target's definition of "brief" for a given schema, it should not assume that any particular elements are included.
The FOLIO Z39.50 server recognises element-sets "f" and "b", the lower-case equivalents of the standard element-set names; I will add support for the standard names as well as these.