Unexpected mission behaviour

last night we rolled a team mission with one of my guildmates in Tir, QL 26. We are both around level 16. I play a trader (a bit tweaked) with approximatively 500 AC (carbonum QL 24).
What's happend is that every mob we met in the mission was unaffected at all by our buffs.

1) Neither my deprive nor ransack lowered their fire power.
2) Usually i can evade some of the attacks, in that case no evade at all was available. i was hit each time (between 22 and 36 depending of the mob)
3) their rate of fire was less than one second and my armour AC seemed to be inneficient against their attack.

We finaly went out ,unable to progress into the mission. My teamate logged his Enforcer lvl 105 and he entered the mission with his full buffs. Against the same mobs he never evaded a single shot and was hit each time around 36 dmgs.

I would like to know if someone else experienced such a problem in team or solo mission. unfortunately it was too late for us to file a petition.