Just thought I'd get this down as all I found on the net failed to help me.
A while ago there was a windows 8 update that caused machines to crash. Said update also stopped Synergy servers from sending the shift key to clients. See the link here for information on that.
github discussion about windows update k27blahblah
None of the workarounds or fixes worked for me as I'm currently using Windows 10 tech preview.
So I found a solution:
Either switch to the linux machine as the server...
On whichever distro you're using workout how to assign keys to a command. Then pick two keys you don't think you'll need as much as shift..
Make sure you have xdotool installed:
sudo apt-get install xdotool
Open the keyboard shortcut settings menu
Add a shortcut and assign it to a key, naming it shift_up.
In the 'command' field type:
xdotool keydown Shift
Repeat steps 4 and 5 for a shift up key using 'keyup' instead of 'keydown'
I did this in mint. It will have a similar equivalent in Ubuntu and is definitely easily figured out in Suse etc.
If your machine is quick you could write a shell script and point the shortcut to it which has an if else statement so you can use just one key.