PPTP Problème de connexion

Bonjour,

Lorsque je me connecte a mon VPN PPTP present sur mon serveur dédié, j'obtient une erreur windows 629 ou windows 619,
avec pour log serveur :

Dec 24 16:58:43 ks301295 pptpd[20030]: CTRL: Client 89.156.105.180 control connection started
Dec 24 16:58:43 ks301295 pptpd[20030]: CTRL: Starting call (launching pppd, opening GRE)
Dec 24 16:58:43 ks301295 pppd[20031]: Plugin /usr/lib/pptpd/pptpd-logwtmp.so loaded.
Dec 24 16:58:43 ks301295 pppd[20031]: pppd 2.4.5 started by root, uid 0
Dec 24 16:58:43 ks301295 pppd[20031]: Using interface ppp0
Dec 24 16:58:43 ks301295 pppd[20031]: Connect: ppp0 <--> /dev/pts/1
Dec 24 16:58:43 ks301295 pptpd[20030]: GRE: Bad checksum from pppd.
Dec 24 16:59:01 ks301295 /USR/SBIN/CRON[20037]: (root) CMD (/usr/local/rtm/bin/rtm 4 > /dev/null 2> /dev/null)
Dec 24 16:59:13 ks301295 pppd[20031]: LCP: timeout sending Config-Requests
Dec 24 16:59:13 ks301295 pppd[20031]: Connection terminated.
Dec 24 16:59:13 ks301295 avahi-daemon[2650]: Withdrawing workstation service for ppp0.
Dec 24 16:59:13 ks301295 pppd[20031]: Modem hangup
Dec 24 16:59:13 ks301295 pppd[20031]: Exit.
Dec 24 16:59:13 ks301295 pptpd[20030]: GRE: read(fd=6,buffer=610d20,len=8196) from PTY failed: status = -1 error = Input/output error, usually caused by unexpected termination of pppd, check option syntax and pppd logs
Dec 24 16:59:13 ks301295 pptpd[20030]: CTRL: PTY read or GRE write failed (pty,gre)=(6,7)
Dec 24 16:59:13 ks301295 pptpd[20030]: CTRL: Reaping child PPP[20031]
Dec 24 16:59:13 ks301295 pptpd[20030]: CTRL: Client 89.156.105.180 control connection finished

Cordialement,