OK, now I have more clue of that “issue”. I don’t believe that’s a urbackup problem, I’m doing something wrong, or don’t do at all.
I modified my my iptables idea totally, cause simple rules wasn’t enough. So I run chains (for interested configs below).
I have two ub machines. first, URBACKUP1 which i use to backup common devices, users pc’s laptops etc.
Second machine, URBACKUP2 is used to backup more critical machines for me.
Naturally there is much more common devices in my network than critical, so my idea to configure firewall is simple.
URBACKUP1 - block traffic to/from criticals, allow the rest
URBACKUP2 - Allow Criticals, block the rest.
So, URBACKUP1 in this config work as intended, it scan network, find devices in it, and just works.
The “problem” is with URBACKUP2, as I mentioned above, it dont see criticals until they aren’t pointed as a client hint. If I remove them from Hints list, they go offline for server, put on the list, they magically show up 
The fun thing is:
# tcpdump -G 3600 --interface=em1 -n -w /home/Downloads/tcpdump-R420-fw-on.pcap
tcpdump: listening on em1, link-type EN10MB (Ethernet), capture size 262144 bytes
41725 packets captured
41773 packets received by filter
0 packets dropped by kernel
made when backups weren’t going on. In time of tcp dump I made that Hint list magic 
but I see 50 packets wasn’t caught by tcpdump,
Below fw configs, maybe someone knows what I missed?
Maybe someone will find his own use of it : - )
URBACKUP2.txt (2.8 KB)