Forgive me, all, for possibly repeating what someone else has said on another page but, frankly, I can't be bothered to read all 10 pages.
I think the real question, from me, is why this has suddenly started happening?
For me it's started in the last two weeks. Now almost without rhyme and reason it seems to occur. Almost every other time I connect to sync I get a message saying my computer is not authorised, which it is. I try to authorise and of course every time without fail get the 42408 error.
I tried the SC Info folder workaround and once that worked and another time I was forced to restore my iPod which I could have done without as that usually requires 5 restores before it sorts itself out.
I remember a couple of years when, after another upgrade, a similar problem occurred when you couldn't sync any purchased songs. This went on for too long without so much as a workaround.
I am beginning to feel the same way about Apple as some feel about Microsoft. Their product(s) cause me so much hassle it's almost not worth it. When they work, great, but when they don't it's just painful to have to resolve the issue. With the faulty hardware and buggy software upgrades what are great little gadgets, conceptually, are in practice monumentally irritating.