I am having a problem where my RB750G will not send a router advertisement. I have to take down the Ethernet interface that I am trying to advertise on, then when I bring it back up then it starts advertising.

When it tries booting the router attempts to advertise the fe80: address, instead of the 2001: address, and the logs say that there was an invalid argument.

Hi, look at this thread: http://forum.mikrotik.com/viewtopic.php?f=2&t=38166. May be it is a same cause. I had problems with IPv6 and RA. Finally last week i have found some workaround.

It looks like it may be the same or a similar problem, but I am using the built in switch chip.

Hm, I tried to use a internal switch, but I noticed serious issues with Queues. Simply Queue Tree was unable to keep enough bandwidth for voip calls. So I drop switch and use bridge.