I get that sometimes. It's just regular BNet lag. Try logging into your Account again. That usually does it for me. On the other hand, do you get this error message more than once in a row? Usually after I get it once, it sorts itself out.
As you said it expees soon, I guess you haven't played much so far. Perhaps you started the necro, played him for a short amount of time and for some reason a rollback was made on the server, i.e. it was brought back to an older state of the game and corresponding data files. If that old system state didn't hold your necromancer so far, but the account overview still does (e.g. because the rollback didn't affect the part wehere account info is saved), a message like that should appear.
If that's the case, you necro doesn't exist anymore which might mean that everything you muled to him is gone as well. However, it might be worth to vcheck the characters from which you muled stuff to him. Perhaps it's on them again.