If you encounter the following error during VPN connection:
pptp: nm-pptp-service-12543 warn[decaps_gre:pptp_gre.c:331]: short read (-1): Message too long
there's an easy fix. You have to lower your MTU (automatically obtained value was invalid).
First, you have to locate your VPN gateway address in syslog:
NetworkManager: <info> VPN Gateway: X.X.X.X
Then, you have to check minimum MTU toward this address:
$ traceroute –mtu X.X.X.X
traceroute to X.X.X.X (X.X.X.X), 30 hops max, 65000 byte packets
1 192.168.43.1 (192.168.43.1) 4.309 ms F=1380 4.042 ms 2.535 ms
2 * *^C
Then you have to change MTU it in your primary connection settings (network manager on Ubuntu below):
That's all!. No more spurious disconnects!