Dear Maintainer, I've just been setting up a l2tp vpn and found that pppd segfaulted whenever I tried to connect. Digging into things, I found this Ubuntu bug report: https://bugs.launchpad.net/ubuntu/+source/ppp/+bug/1410675 ...and I can confirm that removing the option resolved the issue for me. I cannot comment on the analysis offered on the bug report.
Can you still reproduce this bug with ppp 2.4.9? If so, then please provide a stack trace with symbols.