nlrikkert

If the fore mentioned options do not work I have an alternative. Open the console commandline and type 'TCL'. This will put you in to no clip mode, in which you may fly through the first gate. use 'TCL' again to turn it back off and head through the frontdoor. The NPC inside might call out to you for trespassing, just wait a few seconds and Serena should appear and the quest should continue.