I have also hit this problem on an RB2011 running L2TP/IPSec VPN server as described here:
http://forum.mikrotik.com/t/ip-route-cache-bug/70827/1
I obtained this info via the serial console, all other networking is unavailable:
uptime: 2d20h16m
version: 6.10
cache-size: 16384
max-cache-size: 16384
Rolling back to 5.25 fixed the issue, I haven’t had a chance to test 6.11 but MikroTik support said they could re-create my issue and would try have the fix included in it.
Cheers