to prove it.
I put my hands in the air and admit: "It's a fair cop guv". So before you assign me to that particular dustbin reserved for players who have been caught cheating please let me explain myself. The mini instance "Rescue by Moonlight" is on the Epic Quest line so you need to complete it to progress. You have to fight your way the port of Kheledul to rescue Avothol from the Dourhands. It is full of level 11 and 12 elite Dourhands - so at level 15 Throg had no real trouble going through the mission with a fellowship group. Unfortunately due to some hiccup which I don't yet understand I didn't complete the quest despite completing the mission and now I have to do it again. At level 15 Throg can solo a level 12 elite so I thought I'd try going back alone. Moreover I had been through this port prior to the mission and I knew there was a network of canals that would allow me to swim around most of the guards to get straight to the location of Avothol. I snuck into the port, dived into the canal and got the warning message you have just seen.
Pretty impressive in some respects. Turbine have clearly implemented some form of automatic "Exploit Detection" which detects if players are trying to cheat. The detection seems to be based on the fact that I had got into a position where the mob couldn't reach me.
Unfortunately in this case the exploit detection is incredibly immersion breaking - I am trying to sneak into a guarded port by night why on earth would I not try to avoid the guards by swimming along the canal. I should point out that this canal has steps into it so it pretty much invites you to use it.
I can't complain too much because at least I got a warning and an opportunity to rectify the situation before before being booted out but it is a pity this exploit detection is such a blunt instrument.
EDIT: I went through that mission again last night with a fellowship. I managed to complete but another member of my fellowship didn't complete the quest despite going through the whole mission with us and clicking everything. Perhaps this quest is bugged.