While being logged into my OS X dektop via ssh, I tried to connect using
syncergyc -n MacDesktop linuxbox
to a linux box linuxbox running the synergy server. It obviously didn't work and I found a lot of errormessages of the kind
/build/buildd/synergy-1.4.12/src/lib/server/CClientListener.cpp,147 2015-03-06T12:24:05 ERROR: invalid message from client "MacDesktop": DINF
searching the web led me to this page which indicated, that the screensaver might be the problem: Right! After disabling the screensaver by moving the physically attached trackpad, the connection tried above worked without problems.
With Barrier 2.1.0-RELEASE-8b69f9fe on OS X 10.10 as server and version Version: 2.3.2+dfsg-1build1 on Linux Mint 20 als client you need the following on the servers (i.e. Macs) config in Linux Mints screen section:
alt = altgr