Problem with Ethernet port dropping every 5 minutes

I have two MT routers connected together via a wireless bridge.
Every 5 minutes all traffic over the link is interrupted.
If I look in the logs one router shows the message: “ether5: bridge port received packet with own address as source address (00:0c:42:8a:43:56), probably loop”
However there’s no possibility of a loop in our topology as there are no alternate routes.
The network has many more identical links and they work fine. Just this one has the problem.

Any ideas on how to resolve / fix?