I apologize for spamming with the message above; I was just confused after solving the issue with the lease for the bridge. However, after investigating and searching for what could be causing this problem, I found that the solution might be what you mentioned earlier:
I did this, but it didn’t help. I also found a post here on the forum by @kobuki, with the difference that I don’t have the problematic comment in the logs:
http://forum.mikrotik.com/t/weird-warning-with-bridge-config-regarding-vlans/150103/1
My current port configuration is set to “admit only VLAN tagged” for Frames Type, which matches my configuration. All bridged VLANs are set separately, but the problem persists.