Might be the same problem and solution described here:
http://lists.xymon.com/archive/2013-September/038150.html
[Mon Nov 20 19:36:25 2017] [error] [client 10.116.131.209] 2017-11-20
19:36:25.250081 , referer: http://adgtbigbrother/xymon-
cgi/showgraph.sh?host=promph1.analog.com&service=la&graph_
width=576&graph_height=120&disp=promph1.analog.com&
nostale&color=green&graph_start=1511004887&graph_end=
1511177687&action=menu
[Mon Nov 20 19:36:25 2017] [error] [client 10.116.131.209] Sendto failed:
Connection refused, referer: http://adgtbigbrother/xymon-
cgi/showgraph.sh?host=promph1.analog.com&service=la&graph_
width=576&graph_height=120&disp=promph1.analog.com&
nostale&color=green&graph_start=1511004887&graph_end=
1511177687&action=menu