My BIG Trouble Online only for few minutes

Sorry for the name of the topic but I really don’t know how to named it.
the problem is that I have many mikrotiks working as a vpn servers.
Well I see that many of my mikrotiks disconnect people on a certain time like 00:08:56 every session is with this continuance. But I can’t find the problem it happened only on several mikrotiks for several people and there is nothing in common with them even the time.for example
user:rumqna_r disconnect in 00:08:56 and user stefan disconnects is 00:21:05 ???

log:
12:20:56 pptp,debug,packet rcvd Echo-Request from 192.168.113.71
12:20:56 pptp,debug,packet identifier=150994944
12:20:56 pptp,debug,packet sent Echo-Reply to 192.168.113.71
12:20:56 pptp,debug,packet identifier=150994944
12:20:56 pptp,debug,packet result-code=1
12:20:56 pptp,debug,packet error-code=0
12:20:56 pptp,ppp,debug <192.168.113.71>: PPP disconnected
12:20:56 pptp,ppp,debug <192.168.113.71>: LCP lowerdown
12:20:56 pptp,ppp,debug <192.168.113.71>: LCP closed
12:20:56 pptp,ppp,debug <192.168.113.71>: CCP lowerdown
12:20:56 pptp,ppp,debug <192.168.113.71>: IPCP lowerdown
12:20:56 pptp,ppp,debug <192.168.113.71>: IPCP closed
12:20:56 pptp,ppp,info,account rumqna_r logged out, 538 490180 8515717 5468 7145
12:20:56 pptp,ppp,info,account rumqna_r logged out, 538 490180 8515717 5468 7145
12:20:56 pptp,ppp,info <pptp-rumqna_r>: terminating… - disconnected
12:20:56 pptp,ppp,info <pptp-rumqna_r>: terminating… - disconnected
12:20:56 pptp,ppp,debug <192.168.113.71>: PPP destroy
12:20:56 pptp,ppp,debug <192.168.113.71>: PPP stopped
12:20:56 pptp,ppp,info <pptp-rumqna_r>: disconnected
12:20:56 pptp,ppp,info <pptp-rumqna_r>: disconnected
12:20:56 pptp,ppp,debug <192.168.113.71>: CCP lowerdown
12:20:56 pptp,ppp,debug <192.168.113.71>: CCP down event in starting state
12:20:56 pptp,ppp,debug <192.168.113.71>: IPCP lowerdown
12:20:56 pptp,ppp,debug <192.168.113.71>: IPCP down event in starting state