Have you tried reassigning the default shader to the object? I know in at least one case my shader network got messed up.
Deleting UV's before mapping is never really a good idea, whatever the case! :-)
Not sure what the error means. If your material is just a plain Lambert, then something is probably wrong with your object. If it's a more complex shader (ie with bump maps, ramps, &c attached), then I'll wager it's your shader...
"Ad astra per aspera..."