I found sollution after one hour of play this quest: Save before exit Coracavus, clear all small Venatori campc, kill mages in Cassandra side quest, kill giants near the both gates and close both rifts. Maybe you dont have to do this all of it, but doing so should help, and upon entering Echobank Fort Servis should appear near Archer when Lazurite ore can be mined.
EDIT:
I do some tests.
When i was tryind find sollution first time i was loading save in Coracavus and always when i go to Echobank fort Servis wont appear.
And one time when i wasn't going to Echobank fort to search him-when i was about kill everything in Canyon(see above) when i get there after crearing whole canyon he was standing in Fort waiting for me.
Now when i load save file from Coracavus and go straigh to Echobank Fort everytime Servis is there waiting for me- i checked about 20 times.
It seems all you have to do is just save before entering Echabank canyon(In Coracavus), go straigh to Echobank Fort and pray he is there., if hes not repeat.
I play on PS3, i hope it helps You guys who got the same bug🙂
