Comment # 6 on bug 957961 from Dominique Leuenberger

(In reply to Marcus Meissner from comment #4)
> hmm, we also need to update the NetworkManager-pptp package. doing that...

(checked a TW setup - might look slightly different on 13.1)

I found those packages that have a hard version dependency on ppp:
NetworkManager (released)
NetworkManager-pptp (now in release process)
capi4linux (package i4l-base) 
ppp-userpass (package i4l-base)

Might be worthy to check if those are still installable post ppp update.