The soldier that you need to talk to start the quest is actually at Delphe Castle. He's on your right if you are heading through the tunnel from Calpheon to Heidel.

Travel to the place where the objectiv is and inside the tent its 3 guards, talk to the guard at the other side off the table. might have to talk to all three inside there but you figure it out
had some problem finding him cause i though he was at the castle , he was not.

Cannot seem to start this quest.
I checked all quests from the questline u get before this quest, but no luck.
Even on my first character i cannot see it and yes ive all icons on in the Quest Tab.

I can't find the soldier with the quest. Can someone explain it further or add a screenshot. This would be very nice.

EDIIT: could accept and finish the quest after patch on 05-04-2017, they fixed the issue (questgiver like Razlym mentioned)