Using 2.2.3 on SUN/solaris and PC/suse-linux in a forward proxy configuration.
Without other http connections the ftp functionality is fine. With other http
connections the ftp starts to fail increasingly under http request load until it
no longer functions. This is easily reproduced using one http connection/sec
through the forward proxy. Although proxy_util.c(2141) says the connection is
complete there are no connection attempts from the proxy to the destination
server. This was checked with snoop/tcpdump. In the example below note the
hostname as www.gatesec.net for the ftp.suse.org address (195.135.221.132).

Successful connections show this:
Sep  5 23:34:48 hereford fproxy1: [Tue Sep 05 23:34:48 2006] [debug]
mod_proxy_ftp.c(894): proxy: FTP: connecting ftp://ftp.suse.com/ to ftp.suse.com:21
Sep  5 23:34:48 hereford fproxy1: [Tue Sep 05 23:34:48 2006] [debug]
proxy_util.c(2141): proxy: FTP: connection complete to 195.135.221.132:21 ((null))

Unsuccessful connections show this when using one connection/sec to
http://www.gatesec.net:
Sep  5 23:45:19 hereford fproxy1: [Tue Sep 05 23:45:19 2006] [debug]
mod_proxy_ftp.c(894): proxy: FTP: connecting ftp://ftp.suse.com/ to ftp.suse.com:21
Sep  5 23:45:19 hereford fproxy1: [Tue Sep 05 23:45:19 2006] [debug]
proxy_util.c(2141): proxy: FTP: connection complete to 195.135.221.132:21
(www.gatesec.net)
Sep  5 23:45:19 hereford fproxy1: [Tue Sep 05 23:45:19 2006] [debug]
mod_proxy_ftp.c(981): proxy: FTP: control connection complete
Sep  5 23:45:21 hereford fproxy1: [Tue Sep 05 23:45:21 2006] [debug]
mod_proxy_ftp.c(626): proxy:<FTP: 4294967295 <unable to read result>
Sep  5 23:45:21 hereford fproxy1: [Tue Sep 05 23:45:21 2006] [debug]
proxy_util.c(1816): proxy: FTP: has released connection for (*)
Sep  5 23:45:21 hereford fproxy1: [Tue Sep 05 23:45:21 2006] [error] [client
127.0.0.1] proxy: Error reading from remote server returned by ftp://ftp.suse.com/
Sep  5 23:45:21 hereford fproxy1: 127.0.0.1 - - [05/Sep/2006:23:45:19 +1000]
"GET ftp://ftp.suse.com/ HTTP/1.1" 502 67 "-" "Mozilla/5.0 (X11; U; Linux i686;
en-US; rv:1.8.0.5) Gecko/20060719 Firefox/1.5.0.5"

/opt/chroot/fproxy-2.2.3/bin/httpd -l
Compiled in modules:
  core.c
  mod_authn_file.c
  mod_authn_default.c
  mod_authz_host.c
  mod_authz_groupfile.c
  mod_authz_user.c
  mod_authz_default.c
  mod_auth_basic.c
  mod_include.c
  mod_filter.c
  mod_log_config.c
  mod_logio.c
  mod_env.c
  mod_headers.c
  mod_proxy.c
  mod_proxy_connect.c
  mod_proxy_ftp.c
  mod_proxy_http.c
  mod_proxy_ajp.c
  mod_proxy_balancer.c
  worker.c
  http_core.c
  mod_status.c
  mod_cgid.c
  mod_dir.c

/opt/chroot/fproxy-2.2.3/bin/httpd -V
Server version: Apache/2.2.3
Server built:   Sep  5 2006 22:33:20
Server's Module Magic Number: 20051115:3
Server loaded:  APR 1.2.7, APR-Util 1.2.7
Compiled using: APR 1.2.7, APR-Util 1.2.7
Architecture:   32-bit
Server MPM:     Worker
  threaded:     yes (fixed thread count)
    forked:     yes (variable process count)
Server compiled with....
 -D APACHE_MPM_DIR="server/mpm/worker"
 -D APR_HAS_SENDFILE
 -D APR_HAS_MMAP
 -D APR_HAVE_IPV6 (IPv4-mapped addresses enabled)
 -D APR_USE_SYSVSEM_SERIALIZE
 -D APR_USE_PTHREAD_SERIALIZE
 -D SINGLE_LISTEN_UNSERIALIZED_ACCEPT
 -D APR_HAS_OTHER_CHILD
 -D AP_HAVE_RELIABLE_PIPED_LOGS
 -D DYNAMIC_MODULE_LIMIT=128
 -D HTTPD_ROOT="/fproxy-2.2.3"
 -D SUEXEC_BIN="/fproxy-2.2.3/bin/suexec"
 -D DEFAULT_SCOREBOARD="logs/apache_runtime_status"
 -D DEFAULT_ERRORLOG="logs/error_log"
 -D AP_TYPES_CONFIG_FILE="conf/mime.types"
 -D SERVER_CONFIG_FILE="conf/httpd.conf"