We had this problem on our server, make sure you space all your ports by at least 4
for example, you now use 55001, 55002, 55003. 5501,5502,5503. 27001,27002,27003 etc.
Instead use: 55001,55005,55009. 5501,5505,5509. 27001,27005,27009
also dont forget to forward the port thats located inside the DefaultGameUsersettings.ini most ppl overlook that one.
its located at shootergame/config.
This should solve all your problems
with regards
Ossiwozzy