Ive had this same problem recently, and I have tried the retargeting thing and that doesnt work for me.
I think the issue might have to do with lag... but it might also be an issue with pathing.
Here is what i have discovered kiting wraiths in south badlands with my bard:
If im kiting on a level surface it will happen MUCH less often than a hilly surface, but it will still happen.
Ive noticed that when I get that message the mob appears to be stopped in one place and not moving, and I will continually get the "noone around here with that name" message until I let the mob hit me, sometimes i have to let it hit me more than once. Once the mob has hit me it will start moving normally and i can cast again. I have let the mob hit me two different ways:
1) I run up to it where it has appeared to stop and let it maul me or
2) stand in place, and let the mob "warp" to me and hit me, if i stay in one place the mob will eventually warp to me and hit me, then ill be able to cast normally again
Both methods have the same effect.
/Adefan