I have seen a number of other posts describing the exact same issue that I am now experiencing.
When in Ravensthorpe I activate the River Raid quest ‘The Ulfberht Sword’. Doing so prompts me to visit the River Rhine however, when I arrive there, the quest completely disappears from my quest log. Therefore, when I travel to the 2 fortified locations that are supposed to be where you find the military captains, they are nowhere to be found. I have reloaded the game several times, I have visited and completed each raiding location on the river numerous times but to no avail.
The quest disappears from my log as soon as I get to the River Rhine and so the captains are not at their designated locations because the quest is no longer ‘active’ so to speak.
So frustrating!! Please help.