In case anyone runs into this issue again and finds this old post with searches, I wanted to provide a solution for anyone encountering OP's issue.
Last night a steam user ran into this same issue: https://steamcommunity.com/app/48700/discussions/6/1699415798774207985/
Since athletics skill determines "sneaking" ability in Viking Conquest, he tried reducing his athletics temporailly using import/export, so more enemies would be quickly alerted to his presence.
This solved the issue, and some hidden enemies came rushing out at him when he tried the mission again, and the quest was completed as normal.
A player who runs into the issue should consider following Kalarhan's advice above and officially reporting it with a saved game in support forum, so it can potentially be fixed in an update. The Steam User I linked can not officially report it as he states he is using VC Balance Mod 4.1, and testing with a vanilla installation is required to report bugs.
But for a personal workaround, you should be able to also solve the issue for yourself and continue playing by using import/export and trying the mission with lower athletics skill.
