CTRL: PTY read or GRE write failed (pty,gre)=(6,7)

GRE: xmit failed from decaps_hdlc: Operation not permitted

CTRL: PTY read or GRE write failed (pty,gre)=(6,7)

Jan 10 22:52:54 vpn12 pptpd[18635]: GRE: read(fd=6,buffer=611860,len=8196) from PTY failed: status = -1 error = Input/output error, usually caused by unexpected termination of pppd, check option syntax and pppd logs

Jan 10 22:52:54 vpn12 pptpd[18635]: CTRL: PTY read or GRE write failed (pty,gre)=(6,7)

Run the following:

iptables -A INPUT -p 47 -j ACCEPT

GRE is protocol 47 so you need to make sure you allow it.