The coordinates are bogus - they point to Nøklevann just outside AG#14. To find this bonus cache, you need to collect clues from four other caches in Østmarka. All these are designed for and perfect to take while skiing, but will be available all year.
This is THE AG#15 Østmarka multi-cache bonus. The clues can be found in the following caches:
You can first log this after you have visited the others. It is placed less then a kilometer from AG#14, so it can be found after finding the final clues. Both are designed to so they can be visited during evenings (after work, etc.), since they are close to the illuminated tracks. Be careful when you retrieve the cache so mugglers don't see you, and try to cover/confuse your tracks.
The cache can be found at the following location:
N 59° 5 f. [ k-c-i ] [ d+f ] [ e-f ] .. E 010° 5 [ g-c ]. [ c+i ] [ d+f ] [ h-j ]
NB! All multiplications are marked with ' * ', so none here
The cache is a small lock-n-lock box, containing a combined micro stashnote and log, pencil and some tradeables. It is now winter secured and can be found on skies, close to groomed ski tracks. Enjoy!