NOSRV errors seen in haproxy logs - haproxy

Question

We have haproxy in front of 2 apache servers and every day for less than a minute I am getting NOSRV errors in haproxy logs. There are successful requests from the source IP so this is just intermittent. There is no entry of any error in the backend logs.
Below is the snippet from access logs:
Dec 22 20:21:25 proxy01 haproxy[3000561]: X.X.X.X:60872 Local_Server~ Local_Server/<NOSRV> -1/-1/-1/ -1 0 0 0 {} "POST /xxxxtransaction HTTP/1.1" 0 0
Dec 22 20:21:26 proxy01 haproxy[3000561]: X.X.X.X:43212 Local_Server~ Local_Server/<NOSRV> -1/-1/-1/ -1 0 0 0 {} "POST /xxxxtransaction HTTP/1.1" 0 0
Dec 22 20:21:26 proxy01 haproxy[3000561]: X.X.X.X:43206 Local_Server~ Local_Server/<NOSRV> -1/-1/-1/ -1 0 0 0 {} "POST /xxxxtransaction HTTP/1.1" 0 0
Dec 22 20:21:26 proxy01 haproxy[3000561]: X.X.X.X:60974 Local_Server~ Local_Server/<NOSRV> -1/-1/-1/ -1 0 0 0 {} "POST /xxxxtransaction HTTP/1.1" 0 0
Dec 22 20:21:27 proxy01 haproxy[3000561]: X.X.X.X:32772 Local_Server~ Local_Server/<NOSRV> -1/-1/-1/ -1 0 0 0 {} "POST /xxxxtransaction HTTP/1.1" 103 0
Dec 22 20:21:27 proxy01 haproxy[3000561]: X.X.X.X:32774 Local_Server~ Local_Server/<NOSRV> -1/-1/-1/ -1 0 0 0 {} "POST /xxxxtransaction HTTP/1.1" 59 0
Dec 22 20:21:27 proxy01 haproxy[3000561]: X.X.X.X:32776 Local_Server~ Local_Server/<NOSRV> -1/-1/-1/ -1 0 0 0 {} "POST /xxxxtransaction HTTP/1.1" 57 0
below is the HAproxy config file:
defaults
log global
timeout connect 15000
timeout check 5000
timeout client 30000
timeout server 30000
errorfile 400 /etc/haproxy/errors/400.http
errorfile 403 /etc/haproxy/errors/403.http
errorfile 408 /etc/haproxy/errors/408.http
errorfile 500 /etc/haproxy/errors/500.http
errorfile 502 /etc/haproxy/errors/502.http
errorfile 503 /etc/haproxy/errors/503.http
errorfile 504 /etc/haproxy/errors/504.http
frontend Local_Server
bind *:80
bind *:443 ssl crt /etc/haproxy/certs/
mode http
option httplog
cookie SRVNAME insert indirect nocache maxidle 8h maxlife 8h
#capture request header X-Forwarded-For len 15
#capture request header Host len 32
http-request capture req.hdrs len 512
log-format "%ci:%cp[%tr] %ft %b/%s %TR/%Tw/%Tc/%Tr/%Ta %ST %B %CC %CS %tsc %ac/%fc/%bc/%sc/%rc %sq/%bq %hr %hs %{+Q}r"
#log-format "%ci:%cp %ft %b/%s %Tw/%Tc/%Tr/ %ST %B %rc %bq %hr %hs %{+Q}r %Tt %Ta"
option dontlognull
option http-keep-alive
#declare whitelists for urls
acl xx_whitelist src -f /etc/haproxy/xx_whitelist.lst
acl is-blocked-ip src -f /etc/haproxy/badactors-list.txt
http-request silent-drop if is-blocked-ip
acl all src 0.0.0.0
######### ANTI BAD GUYS STUFF ###########################################
#anti DDOS sticktable - sends a 500 after 5s when requests from IP over 120 per
#frontend for stick table see backend "st_src_global" also
#Restrict number of requests in last 10 secs
# TO MONTOR RUN " watch -n 1 'echo "show table st_src_global" | socat unix:/run/haproxy/admin.sock -' " ON CLI.
#ZZZ THIS MAY NEED DISABLEING FOR LOAD TESTS ZZZZ
# Table definition
http-request track-sc0 src table st_src_global #<- defines tracking stick table
stick-table type ip size 100k expire 10s store http_req_rate(50000s) #<- sets the limit for and time to store IP
http-request silent-drop if { sc_http_req_rate(0) gt 50000 } # drops if requests are greater the 5000 in 5 secs
# Allow clean known IPs to bypass the filter
tcp-request connection accept if { src -f /etc/haproxy/xx_whitelist.lst }
#Slowlorris protection -send 408 if http request not completed in 5secs
timeout http-request 10s
option http-buffer-request
# Block Specific Requests
#http-request deny if HTTP_1.0
http-request deny if { req.hdr(user-agent) -i -m sub phantomjs slimerjs }
#traffic shape
#xxxx.xxxx.xx.xx
acl xxxxx.xxxxx.xx.xx hdr(host) -i xxxx.xxxx.xx.xx
use_backend xxxx.xxxx.xx.xx if xxxx.xxxx.xx.xx xx_whitelist #update from proxys
#sticktable for dos protection
backend st_src_global
stick-table type ip size 1m expire 10s store http_req_rate(50000s)
backend xxxxxxx.xxxxx.xx.xx
mode http
balance roundrobin
option forwardfor
http-request set-header X-Forwarded-Port %[dst_port]
http-request add-header X-Forwarded-Proto https if { ssl_fc }
server web01-http x.x.x.x:80 check maxconn 100
server web03-http x.x.x.x.:80 check maxconn 100