VPN PPTPD

Ho creato una VPN con PPTPD con Debian Stable. Per un po' è andato tutto bene, poi ad un certo punto a smesso di funzionare regolarmente.
In particolare mi sono accorto che non funziona con client Windows, perché creando una connessione con un router invece funziona perfettamente.
La configurazione è semplicissima, non ci sono opzioni particolari. Allego il log del server quando provo a connettermi.
Grazie

Mar 5 17:12:06 backup pptpd[16827]: CTRL: Client 87.3.210.161 control connection started
Mar 5 17:12:06 backup pptpd[16827]: CTRL: Starting call (launching pppd, opening GRE)
Mar 5 17:12:06 backup pppd[16828]: Plugin /usr/lib/pptpd/pptpd-logwtmp.so loaded.
Mar 5 17:12:06 backup pppd[16828]: pptpd-logwtmp: $Version$
Mar 5 17:12:06 backup pppd[16828]: pppd 2.4.4 started by root, uid 0
Mar 5 17:12:06 backup pppd[16828]: using channel 28
Mar 5 17:12:06 backup pppd[16828]: Using interface ppp0
Mar 5 17:12:06 backup pppd[16828]: Connect: ppp0 <--> /dev/pts/1
Mar 5 17:12:06 backup pppd[16828]: sent [LCP ConfReq id=0x1 ]
Mar 5 17:12:06 backup pptpd[16827]: GRE: Bad checksum from pppd.
Mar 5 17:12:09 backup pppd[16828]: sent [LCP ConfReq id=0x1 ]
Mar 5 17:12:12 backup pppd[16828]: sent [LCP ConfReq id=0x1 ]
Mar 5 17:12:15 backup pppd[16828]: sent [LCP ConfReq id=0x1 ]
Mar 5 17:12:18 backup pppd[16828]: sent [LCP ConfReq id=0x1 ]
Mar 5 17:12:21 backup pppd[16828]: sent [LCP ConfReq id=0x1 ]
Mar 5 17:12:24 backup pppd[16828]: sent [LCP ConfReq id=0x1 ]
Mar 5 17:12:27 backup pppd[16828]: sent [LCP ConfReq id=0x1 ]
Mar 5 17:12:30 backup pppd[16828]: sent [LCP ConfReq id=0x1 ]
Mar 5 17:12:33 backup pppd[16828]: sent [LCP ConfReq id=0x1 ]
Mar 5 17:12:36 backup pppd[16828]: LCP: timeout sending Config-Requests
Mar 5 17:12:36 backup pppd[16828]: Connection terminated.
Mar 5 17:12:36 backup pppd[16828]: Modem hangup
Mar 5 17:12:36 backup pppd[16828]: Exit.
Mar 5 17:12:36 backup pptpd[16827]: GRE: read(fd=6,buffer=610a80,len=8196) from PTY failed: status = -1 error = Input/output error, usually caused by unexpected termination of pppd, check option syntax and pppd logs
Mar 5 17:12:36 backup pptpd[16827]: CTRL: PTY read or GRE write failed (pty,gre)=(6,7)
Mar 5 17:12:36 backup pptpd[16827]: CTRL: Reaping child PPP[16828]
Mar 5 17:12:36 backup pptpd[16827]: CTRL: Client 87.3.210.161 control connection finished