So this has happened twice so far. The first was with the Dark brotherhood, I was supposed to return to Muiri to collect my reward but her arrow shows her at the very top of the map...where I can't go. The 2nd time was at High Hrothgar to learn the 3rd party of unrelenting force from Wulfgar but it shows his arrow at the very bottom of the map where I cannot go. Can someone please help me??

I had the same problem with Danica. No idea about a fix, nothing I did corrected the issue. No idea if it's Mod-related either as I haven't played a regular game further than arriving at Whiterun yet.