IPod Touch (4th gen) will not sync music after the latest iTunes update

Question

I'm using iTunes on a PC with Windows 7 and after the latest update (iTunes 11.2.2), it will not sync with my iPod touch 4 (also updated to the latest possible version - iOS 6.1.6).
Before the update there was no problem with the sync. And this iPod has always been synced only with this computer, and this iTunes library.
After the iTunes update, several songs on my iPod were greyed out and unplayable (the library plays fine in iTunes).
Now, when I try to sync, iTunes goes through all the steps, saying it finished syncing, without any error message but there is no actual change on the iPod.
I've tried deleting all the music on the iPod, to resync the entire music library anew.
Again, iTunes went through all the steps, finished "syncing" and nothing - and, of course, all the music on my iPod is now missing.
When I check the "On this iPod" option in iTunes, the entire song list shows up there but appears greyed out and unplayable.
On the actual iPod device, there is absolutely nothing in the Music app - no songs, no playlists.
The iPod always shows "no data" in the Music app.
Sometimes iTunes shows that I have 18GB of music on the device, sometimes it shows 0GB ...at one point it showed "no space available". But it has plenty of space - 23.5 GB of available space out of 32GB.
To fix this I've tried the following:
- automatic sync, manual sync, manually adding/deleting songs on the iPod, autofilling and replacing songs;
- resetting my iPod by holding the (on/off and home buttons) and resetting through the "reset all settings" option on my iPod;
- reset and restore from backup;
- uninstalled and re-installed iTunes on my PC.
- I've run the diagnostics tool in iTunes - all tests passed;
...plus various other suggested solutions I have found in the similar topics around here.
(I've checked and there are no restrictions to the songs, the computer is authorized, etc.)
Absolutely nothing works and it is very frustrating, as I usually only use the iPod for the music.
Please help.
Violet

Answer 1

Hello, VvioletT. 
Thank you for visiting Apple Support Communities.
I see you are experiencing issues syncing your iOS device.  Here are a couple articles that I would recommend going through when experiencing this issue.
iOS: Troubleshooting USB-related alerts when syncing
http://support.apple.com/kb/TS5254
Resolve issues between iTunes and security software
http://support.apple.com/kb/ts3125
Cheers,
Jason H.