>> Clients coming from
>> 1500 MTU connections to the same m0n0wall work fine. So...
>> unfortunately these two cases have absolutely nothing in common,
>> setup-wise, it seems.
> I think I'll roll back the FW to 1.11 to verify this.
FYI I've downgraded m0n0 to 1.11 and PPTP works fine now. So that
rules out any hardware, connectivity, client or config variables.