pptpd Connection gets stuck after 1 to 2 minutes

I have set up pptpd on my ubuntu machine. Users can connect to VPN-server and have access to internet and all local machines. But after 1 or 2 minutes connection, they even can't ping anything

/etc/ppp/pptpd-options

name vpn
refuse-pap
refuse-chap
refuse-mschap
require-mschap-v2
mppe-stateful
ms-dns 8.8.8.8
proxyarp
nodefaultroute
lock
nobsdcomp
novj
novjccomp
nologfd

/etc/pptpd.conf

option /etc/ppp/pptpd-options
logwtmp
localip 192.168.1.234-250
remoteip 192.168.0.234-250

The same issue on my another Debian machine

EDIT: I tried to ping client in loop. When client does nothing, ping OK. When he begins to load web-sites or another client tries to get shared files of first client through samba, then after 10 seconds about connection gets stuck.

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