VPN randomly stop working

[ikus060]

I have a Unify Gateway with L2TP VPN configured. My users complains about the VPN that stop working and drop connection randomly. 2-3 time a day.

May I get any tips how to investigate or mitigate this issue ?

Sep 16 09:21:47 router xl2tpd[4375]: Maximum retries exceeded for tunnel 30786.  Closing.                                                                                                                                                                                                 
Sep 16 09:21:47 router zebra[753]: interface l2tp0 index 1191 changed <POINTOPOINT,NOARP,MULTICAST>.                                                                                                                                                                                      
Sep 16 09:21:47 router zebra[753]: interface l2tp0 mtu changed from 1400 to 1500                                                                                                                                                                                                          
Sep 16 09:21:47 router kernel: [LAN_IN-4001-D]IN=eth1 OUT=eth0 MAC=fc:ec:da:02:fe:e5:32:b1:f5:5f:ca:61:08:00 SRC=192.168.14.12 DST=82.63.7.75 LEN=48 TOS=0x00 PREC=0x00 TTL=63 ID=10872 PROTO=UDP SPT=58847 DPT=35039 LEN=28                                                              
Sep 16 09:21:53 router pppd[3604]: Connection terminated: no multilink.                                                                                                                                                                                                                   
Sep 16 09:21:53 router zebra[753]: interface l2tp0 index 1191 deleted.                                                                                                                                                                                                                    
Sep 16 09:21:53 router pppd[3604]: Modem hangup