Please attach the debug-level log.

If you're unsure how to obtain it, please do the following (as root):

1.) increase debugging level:
# nmcli g logging level debug

2.) start capturing the log:
# journalctl -fl |tee log.txt

3.) initiate the connection and wait until it terminates

4.) stop logging with CTRL+C

5.) you can now decrease the logging level to info:
# nmcli g logging level info

Please attach the log.txt file,

Also be sure to specify which version of NetworkManger are your running.

Debug here http://pastebin.com/67UVVC5B
My friend gave me an old version 1.0.2-4 networkmanager, this version is stable.
In repositories are now 1.0.4-1,this version closes the connection as shown in the logs.

июл 31 14:29:07 arch pppoe[1508]: Interface enp3s0 has MTU of 1492 -- should be 1500.  You may have serious connection problems.
июл 31 14:29:07 arch NetworkManager[323]: pppoe: Interface enp3s0 has MTU of 1492 -- should be 1500.  You may have serious connection problems.
июл 31 14:29:07 arch NetworkManager[323]: pppoe: Interface enp3s0 has MTU of 1492 -- should be 1500.  You may have serious connection problems.

I'm cherry-picking these two. The latter one should fix the issue:

3751007 device: set Ethernet MTU for PPPoE connections in stage2/config
ecb9385 device: set MTU on IP interface

Please let me know if it works for you.