I upgraded 2 of my boxes yesterday to Router OS 3.4 and seem to be having a problem with the PPPoE Server. After the upgrade everything worked fine, but I notice that after sometime, it kills the PPPoE connections and doesnt allow any new connection on. When I watch the logs to see if any client is trying to connect via PPPoE, all I see it the attempt to connect and it immediately disconnects without trying to authenticate. The only remedy I have found so far, is when it does this, I reboot the unit and all comes back ok for another 2 or 3 hours after which the problem starts again.
Downgrade to 3.3 or 3.2 and see if you still have the issue. Either way, if it occurs again then submit a supout to support@mikrotik.com while the issue is going on.
Sorry I noticed I installed the new version on my RB233 with PCMCIA card and CM9, so if you know where I can get any earlier 3.x version, I would appreciate it. I even noticed that the whole system gets a delay thats why the PPPoE stops working. Even if I open interfaces, it shows me nothing. I also notice pings to the unit get very high.