Additional info:
Regression after migration from f19 to f20.

Looks like selinux:

# grep ntop /var/log/audit/audit.log | audit2allow


#============= ntop_t ==============
allow ntop_t self:capability dac_override;
allow ntop_t snmpd_var_lib_t:dir { read getattr open search };

Could you attach AVCs? (/var/log/audit/audit.log)

type=SERVICE_START msg=audit(1423139958.162:9871): pid=1 uid=0 auid=4294967295 ses=4294967295 subj=system_u:system_r:init_t:s0 msg=' comm="ntop" exe="/usr/lib/systemd/systemd" hostname=? addr=? terminal=? res=success'
type=ANOM_PROMISCUOUS msg=audit(1423139958.340:9872): dev=ens33 prom=256 old_prom=0 auid=4294967295 uid=0 gid=0 ses=4294967295
type=AVC msg=audit(1423139987.076:9873): avc:  denied  { getattr } for  pid=8629 comm="ntop" path="/var/lib/net-snmp" dev="dm-1" ino=2490436 scontext=system_u:system_r:ntop_t:s0 tcontext=system_u:object_r:snmpd_var_lib_t:s0 tclass=dir permissive=1
type=AVC msg=audit(1423139987.076:9874): avc:  denied  { read } for  pid=8629 comm="ntop" name="cert_indexes" dev="dm-1" ino=2490440 scontext=system_u:system_r:ntop_t:s0 tcontext=system_u:object_r:snmpd_var_lib_t:s0 tclass=dir permissive=1
type=AVC msg=audit(1423139987.076:9875): avc:  denied  { open } for  pid=8629 comm="ntop" path="/var/lib/net-snmp/cert_indexes" dev="dm-1" ino=2490440 scontext=system_u:system_r:ntop_t:s0 tcontext=system_u:object_r:snmpd_var_lib_t:s0 tclass=dir permissive=1

This message is a reminder that Fedora 20 is nearing its end of life.
Approximately 4 (four) weeks from now Fedora will stop maintaining
and issuing updates for Fedora 20. It is Fedora's policy to close all
bug reports from releases that are no longer maintained. At that time
this bug will be closed as EOL if it remains open with a Fedora  'version'
of '20'.

Package Maintainer: If you wish for this bug to remain open because you
plan to fix it in a currently maintained version, simply change the 'version' 
to a later Fedora version.

Thank you for reporting this issue and we are sorry that we were not 
able to fix it before Fedora 20 is end of life. If you would still like 
to see this bug fixed and are able to reproduce it against a later version 
of Fedora, you are encouraged  change the 'version' to a later Fedora 
version prior this bug is closed as described in the policy above.

Although we aim to fix as many bugs as possible during every release's 
lifetime, sometimes those efforts are overtaken by events. Often a 
more recent Fedora release includes newer upstream software that fixes 
bugs or makes them obsolete.

Author: Miroslav Grepl <mgrepl>
Date:   Tue Jun 16 12:28:11 2015 +0200

    ntop reads /var/lib/ntop/macPrefix.db and it needs dac_override. It has setuid/setgid. BZ(1058822

selinux-policy-3.13.1-128.2.fc22 has been submitted as an update for Fedora 22.
https://admin.fedoraproject.org/updates/selinux-policy-3.13.1-128.2.fc22

Package selinux-policy-3.13.1-128.2.fc22:
* should fix your issue,
* was pushed to the Fedora 22 testing repository,
* should be available at your local mirror within two days.
Update it with:
# su -c 'yum update --enablerepo=updates-testing selinux-policy-3.13.1-128.2.fc22'
as soon as you are able to.
Please go to the following url:
https://admin.fedoraproject.org/updates/FEDORA-2015-10299/selinux-policy-3.13.1-128.2.fc22
then log in and leave karma (feedback).

selinux-policy-3.13.1-128.2.fc22 has been pushed to the Fedora 22 stable repository.  If problems still persist, please make note of it in this bug report.