Start fop2_server in debug mode and look at the actual connection between the fop2 server and all of your manager_hosts:
service fop2 stop
cd /usr/local/fop2
script capture.log
./fop2_server -X 3
After things settle down, stop capturing:
ctrl-C
exit
service fop2 start
You will have a capture.log file to review, with the actual connection and authorization attempts and their replies.