JimBuna is correct - been there. There is a corruption somewhere caused most likely by the PC. Save your campaign folder then do a clean uninstall of the sim and all folders, then install the sim and one by one your mods and campaign folder and files.
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DID, no cheats, no shortcuts, no non-historic equipment deviations. Boat and crew safety is integral with my immersion style! Follow the historic events during your patrol:
http://www.uboat.net/today.html