I've seen some oddities occur with trade routing that may play into sea lanes being reported as unavailable when it looks like they're open. In my current FotS campaign as Saga, I had Fukuoka as a vassal and a trade agreement via sea lane. Later, I gained the province next to Nagasaki (Saga starting capital) which allowed for a land route to be established. The route subsequently switched to land and I verified that by checking the trade panel icon as well as noting the route itself on the map. However, I noticed that Fukuoka's imports were reporting as unavailable (reddened icons in the trade panel). I was still earning income from the agreement, but they apparently were not. Nor were they trading with anyone else along their newly opened sea lane. I noticed their port was damaged (the one through which our common trade lane formerly ran) and a sea lane still connected with my capital showing 0 income. Apparently the AI export route had not switched to land and the agreement became BOTH land and sea (with me on the lucky side). I could see where a similar occurrence between AI clans might result in sea lanes being reported as unavailable. They actually are unavailable, but there is no way you'd ever see that through the limited UI.