We've recently had to set up a new z3950 connection to a provider who doesn't support element set names. Koha sends 'F' for full in the element set name field at connection time and so we're faced with an error 24 for all searches against this provider. We should make it configurable what element set name is used.. F, B or none at all.
The provider in question updated their z provider to just ignore the set name element, so this is no longer an issue for us.. however I could imagine it coming up down the line some time so I'll leave the bug open but drop the assignee