Console logging
echo: ppp,error,critical 16: Encryption got out of sync - disabling

Nothing in syslog, pptp connectivity lost but the link stays up in ppp

version 6.30.1 (wasn't this bug supposed to be fixed in 6.30)

In my experience this error always occurred when I had the equipment outdated. remember to update both the operating system and the firware.
to check the firware version (which is what most people do not consider) you have to go to system - routerboard - and in it you will see the version running.
to update the firware you can write from the console:
system routerboard upgrade

then restart

system reboot