I encountered this problem when I synced my evernote in Mac OS.
Can't synchronize error, Content is not valid.
I googled and found this in the evernote support forum.
One of the Ruby gems replaces the built-in 'tidy' command in OS X (/usr/bin/tidy) with a broken script in the same location. You need to restore the original /usr/bin/tidy command that comes with OS X.
On my 10.6.6 box, that command is 65072 bytes:
~ $ ls -al
rwxr-xr-x 1 root wheel 65072 Aug 31 2010 /usr/bin/tidy
Once you've fixed that broken part of the OS, Evernote should work ok. You may also want to file a bug with the Ruby folks, who absolutely should not be replacing this file with their own.
It seems like that ruby gems in mac did a mess that replaced the /usr/bin/tidy command unintentionally.
So if your
tidy file size does not match 65072 bytes, just copy on from other Mac OS, and chmod it.