Apr 19 23:11:02 hkd1 pptpd[3076]: GRE: read(fd=7,buffer=608be0,len=8260) from network failed: status = -1 error = Protocol not available
This means that a GRE packet was sent by pptpd on the server, and it was rejected via an ICMP message from the client end, or from a firewall gateway somewhere between your server and the client.
The most common cause of this is that the client is behind a NAT gateway, and the NAT gateway is rejecting GRE unless there is expected traffic because of outbound GRE. But if the server is faster at sending its first GRE packet than the client is, then the gateway won't see outbound GRE before the first inbound GRE packet arrives, and will reject it.
SME server has a configuration option to allow for this case. You can try it.
config setprop pptpd Passive enabled
signal-event remoteaccess-update
With this option, pptpd on the server will wait until it receives a GRE packet before it sends its first GRE packet.