I'm having this problem now.
Tried emptying thrash, clearing cache and checking if the oldest unsynced note had a URL link.
None of the options worked.
I have now some 15 unsynced notes. Apparently in the PC Evernote is working fine, but not from the cell.
It's a good thing that I did notice it without losing any critical data. I'll have to save manually everything before reinstalling the app I guess.