Ipod Nano synching problems with Itunes 11

[lrmmf]

I need your help, i'm having problems synching Nano (can't say its generation, probably the 5th one: 10cm x 3cm, more or less...) with Itunes 11.

I currently have Itunes completely updated, and the error is the same as in the previous versions. It only started happening in the September/November of 2012 - which is strange, i can't recall any big change...

So the error is this: when i sync itunes with my ipod: i get an error like this (translated from my mother language):
"Some elements from itunes library, incluinding "abcde...", weren't copied to ipod "my ipod", because they weren't found".

Naturally, files exist, other way itunes wouldn't have their names.

Regards