There should not be any unexpected error on the mikrotik side, except for when the mikrotik router becomes unresponsive and gives the no buffer space error.
The pptp proxy server I was refering to is not a mikrotik, Its www.mgix.com/pptpproxy
, if that was causing any confusion. There a couple of connections via the proxy to the ROS pptp server.
I can tidy things up at my end, with regards to stopping the clients that are constantly trying to connect to the PPTP server and failing, and see if this prevents the issue, well it would technically just mean that the problem would take longer to appear.