I do experience the same issue as Idiot on Ubuntu 20.04 ("Couldn't connect to Synergy Cloud ...")
Seem to come from the same SSL issue according to the logs:
[ Service ] [2020-12-19T14:25:44] debug: service log file path: /var/log/synergy/synergy-service.log
[ Service ] [2020-12-19T14:25:44] debug: install dir: /usr/bin
[ Service ] [2020-12-19T14:25:44] info: starting service...
[ Service ] [2020-12-19T14:25:44] debug: setting websocket headers, channel=81038 auth=O-U3dotD7A3TJ_zt_E9IN_i0 version=2.0.12-beta check=true
[ Service ] [2020-12-19T14:25:44] debug: initial websoc