nginx ipv6 WAN stops responding
after setting up the system with all the needed hardening (pfs etc.) my server stops awnser http(s) requests over ipv6 originating from WAN after 10-20 minutes, but other local servers have no problems reaching him. the server os is debian 9.5 and all the software is up to date. there are some other servers (debian 9.0) "behind" my sophos utm with nginx and ispconfig with no problems but i can't pinpoint the differences.
used guidline for setup: https://www.howtoforge.com/tutorial/perfect-server-debian-9-nginx-bind-dovecot-ispconfig-3.1/