I use 0 mods, and I have the same issue.
I played for Sturgia as a mercenary, after time passed I joined.
Then I get the endless spam of theese messages.
There have been a few times where its been valid though, where Ragnavad actually offered up 2 of his castles to me, then later he did the same with the newly added clan in 1.54(even removed a fief from Olek).
But after theese issues have been solved, they just keep on spamming endless and when you do hit the button you get the same as on the screenshot.
For me this happends with castles and cities alike, rotating about the things we've captured.
I've yet to have the conditions that others have posted fixed it for them, happend ingame.
We are at peace with all(very odd for Sturgia).
There is 0% intrest in the diplomacy view for declaring war on any of the other factions at the moment.(which is also very odd, typically Battania and Vlandia would have various % intrest, to some degree the Khuzait aswell).