I'm having the exact same issue with the RouterBOARD 3011UiAS on firmware 3.27.

Im' very new to routerOS and I've implemented the following config:
Config:
ether 1: connected with pppoe interface
ether 2: master port
ether 3: slave to 2
ether 4: slave to 2
ether 5: slave to 2
ether 6: master port
ether 7 : slave to 6
ether 8 : slave to 6
ether 9 : slave to 6
ether 10: slave to 6
I'm getting a dynamic IP via PPPoE connected on the ether 1 port. I wanted the rest of the ports to function as a switch an after some reading on the wiki I came up with the following config.
1. to make switch port you need to make 1 master ports and set the other ports to slave on the master port. Because the 3011UiAS has two switch chips I needed to config this with the two ports : eth2 and eth 6.
2. To connect the both switch groups (eth2-5 and eth6-10) I've made a bridge interface and added the both master ports to the bridge (eth 2 and eth 6).
3. Gateway IP assigned to the bridge interface, DHCP, Natting etc..
Is there any flaw in my configuration implementation that could cause the errors ?
-I've made the bridge MAC static
-checked cabling
-port status on bridge ports are designated, default settings rtsp.
-disabled auto negotiation and set the speed and duplex mode static