[Koha] koha on debian 11 / apache 2 and varnish cache

Has anyone tried using varnish cache with koha on debian 11? As my original configration had koha listen on ports 81 (OPAC) and 8081 (administrative), I've amended the apache virtual hosts file to listen on ports 80 and 8080, and set up varnish to respond to ports 81/8081 and forward to ports 80/8080 any requests not in cache. It seems to work, i.e. koha responds on all four ports, when however one tries a Z39.50 query, instead of search results results z3950_search.pl produces the following error message: Error 503 Backend fetch failed Backend fetch failed Guru Meditation: XID: 196655 Varnish cache server Any tips on how to resolve this?