Just got Lumia 521 phones for the family. I found the same problem with the error 8300300B and the sync button greyed out when trying to sync my wife's phone to Itunes. I tried many suggestions from other forums including deleting the iTunes library xml file and the Windows Phone folder in the appdata\local\Microsoft folder.. None worked until I found a file in the C:\users\yourusername\appdata\local\Microsoft\Windows Phone called "lastexception" or something like that, which gave me the clue to the problem. In this text file somewhere, it said "Illegal characters in path." So I went through all the playlists in my Itunes and changed any playlist names which had potentially illegal characters (things like !>:/, don't know about &, but I changed it anyway). Then I deleted the iTunes library xml file and the windows phone folder in app data as suggested in other forums. After doing this, the Windows Phone desktop app read the playlists without an error and the sync button was black. Yea!
I repeated this with my daughter's phone/computer, and all that was necessary was to change the playlist names to eliminate the illegal characters. After changing the playlist names only, her phone synced right up! Only a few hours wasted on Christmas morning, but problem solved. Hope this helps others out there!
The above is completely correct but I'm going to add EVERYTHING I did to solve my sync issues since last year (and that's 3 phones later !! 900 / 920 / 1020)
Here it is, start by doing the above, I removed any 'illegal' characters from my playlist titles - eg Rock/Pop/Indie now reads Rock Pop Indie, take out all characters as pigburner states.
Next, go to control panel and uninstall ANY programmes that would potentially use iTunes, this would include (and I list what I removed) iSyncr, Kes, LG PC Suite, Double Twist, or any third party apps that use iTunes for music.
Next, unplug any external hard drives you may use for back up purposes, eg - I have a 500gb Hitachi hard drive with my iTunes backed up to it
Next, follow this Wiki article link and do EXACTLY as it says,
Fix sync errors between Windows Phone and iTunes (Windows) - Microsoft Community
Next, restart your PC for changes to take effect and connect your phone via the Windows Phone App and complete the new phone registration, obviously selecting iTunes as your library choice.
At this point the Sync button had changed from the ominous GREY tone to a glorious BLACK tone so I selected the playlists (5 to start with, was a bit nervous !) and pressed Sync........
15 mins later it was done......now I'm no techno wizard, far from it and I'm not saying this is the cure but it worked for me. Basically a mash up of every attempt everyone has had to Sync their iTunes so if it works credit the entire Windows-Sphere forums as they all played a part in my solution.
Fingers crossed it works for you, cheers larsy babies